dsa.c

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/*-------------------------------------------------------------------------
 *
 * dsa.c
 *    Dynamic shared memory areas.
 *
 * This module provides dynamic shared memory areas which are built on top of
 * DSM segments.  While dsm.c allows segments of memory of shared memory to be
 * created and shared between backends, it isn't designed to deal with small
 * objects.  A DSA area is a shared memory heap usually backed by one or more
 * DSM segments which can allocate memory using dsa_allocate() and dsa_free().
 * Alternatively, it can be created in pre-existing shared memory, including a
 * DSM segment, and then create extra DSM segments as required.  Unlike the
 * regular system heap, it deals in pseudo-pointers which must be converted to
 * backend-local pointers before they are dereferenced.  These pseudo-pointers
 * can however be shared with other backends, and can be used to construct
 * shared data structures.
 *
 * Each DSA area manages a set of DSM segments, adding new segments as
 * required and detaching them when they are no longer needed.  Each segment
 * contains a number of 4KB pages, a free page manager for tracking
 * consecutive runs of free pages, and a page map for tracking the source of
 * objects allocated on each page.  Allocation requests above 8KB are handled
 * by choosing a segment and finding consecutive free pages in its free page
 * manager.  Allocation requests for smaller sizes are handled using pools of
 * objects of a selection of sizes.  Each pool consists of a number of 16 page
 * (64KB) superblocks allocated in the same way as large objects.  Allocation
 * of large objects and new superblocks is serialized by a single LWLock, but
 * allocation of small objects from pre-existing superblocks uses one LWLock
 * per pool.  Currently there is one pool, and therefore one lock, per size
 * class.  Per-core pools to increase concurrency and strategies for reducing
 * the resulting fragmentation are areas for future research.  Each superblock
 * is managed with a 'span', which tracks the superblock's freelist.  Free
 * requests are handled by looking in the page map to find which span an
 * address was allocated from, so that small objects can be returned to the
 * appropriate free list, and large object pages can be returned directly to
 * the free page map.  When allocating, simple heuristics for selecting
 * segments and superblocks try to encourage occupied memory to be
 * concentrated, increasing the likelihood that whole superblocks can become
 * empty and be returned to the free page manager, and whole segments can
 * become empty and be returned to the operating system.
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/utils/mmgr/dsa.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "port/atomics.h"
#include "storage/dsm.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "utils/dsa.h"
#include "utils/freepage.h"
#include "utils/memutils.h"

/*
 * The size of the initial DSM segment that backs a dsa_area created by
 * dsa_create.  After creating some number of segments of this size we'll
 * double this size, and so on.  Larger segments may be created if necessary
 * to satisfy large requests.
 */
#define DSA_INITIAL_SEGMENT_SIZE ((size_t) (1 * 1024 * 1024))

/*
 * How many segments to create before we double the segment size.  If this is
 * low, then there is likely to be a lot of wasted space in the largest
 * segment.  If it is high, then we risk running out of segment slots (see
 * dsm.c's limits on total number of segments), or limiting the total size
 * an area can manage when using small pointers.
 */
#define DSA_NUM_SEGMENTS_AT_EACH_SIZE 4

/*
 * The number of bits used to represent the offset part of a dsa_pointer.
 * This controls the maximum size of a segment, the maximum possible
 * allocation size and also the maximum number of segments per area.
 */
#if SIZEOF_DSA_POINTER == 4
#define DSA_OFFSET_WIDTH 27     /* 32 segments of size up to 128MB */
#else
#define DSA_OFFSET_WIDTH 40     /* 1024 segments of size up to 1TB */
#endif

/*
 * The maximum number of DSM segments that an area can own, determined by
 * the number of bits remaining (but capped at 1024).
 */
#define DSA_MAX_SEGMENTS \
    Min(1024, (1 << ((SIZEOF_DSA_POINTER * 8) - DSA_OFFSET_WIDTH)))

/* The bitmask for extracting the offset from a dsa_pointer. */
#define DSA_OFFSET_BITMASK (((dsa_pointer) 1 << DSA_OFFSET_WIDTH) - 1)

/* The maximum size of a DSM segment. */
#define DSA_MAX_SEGMENT_SIZE ((size_t) 1 << DSA_OFFSET_WIDTH)

/* Number of pages (see FPM_PAGE_SIZE) per regular superblock. */
#define DSA_PAGES_PER_SUPERBLOCK        16

/*
 * A magic number used as a sanity check for following DSM segments belonging
 * to a DSA area (this number will be XORed with the area handle and
 * the segment index).
 */
#define DSA_SEGMENT_HEADER_MAGIC 0x0ce26608

/* Build a dsa_pointer given a segment number and offset. */
#define DSA_MAKE_POINTER(segment_number, offset) \
    (((dsa_pointer) (segment_number) << DSA_OFFSET_WIDTH) | (offset))

/* Extract the segment number from a dsa_pointer. */
#define DSA_EXTRACT_SEGMENT_NUMBER(dp) ((dp) >> DSA_OFFSET_WIDTH)

/* Extract the offset from a dsa_pointer. */
#define DSA_EXTRACT_OFFSET(dp) ((dp) & DSA_OFFSET_BITMASK)

/* The type used for index segment indexes (zero based). */
typedef size_t dsa_segment_index;

/* Sentinel value for dsa_segment_index indicating 'none' or 'end'. */
#define DSA_SEGMENT_INDEX_NONE (~(dsa_segment_index)0)

/*
 * How many bins of segments do we have?  The bins are used to categorize
 * segments by their largest contiguous run of free pages.
 */
#define DSA_NUM_SEGMENT_BINS 16

/*
 * What is the lowest bin that holds segments that *might* have n contiguous
 * free pages?  There is no point in looking in segments in lower bins; they
 * definitely can't service a request for n free pages.
 */
#define contiguous_pages_to_segment_bin(n) Min(fls(n), DSA_NUM_SEGMENT_BINS - 1)

/* Macros for access to locks. */
#define DSA_AREA_LOCK(area) (&area->control->lock)
#define DSA_SCLASS_LOCK(area, sclass) (&area->control->pools[sclass].lock)

/*
 * The header for an individual segment.  This lives at the start of each DSM
 * segment owned by a DSA area including the first segment (where it appears
 * as part of the dsa_area_control struct).
 */
typedef struct
{
    /* Sanity check magic value. */
    uint32      magic;
    /* Total number of pages in this segment (excluding metadata area). */
    size_t      usable_pages;
    /* Total size of this segment in bytes. */
    size_t      size;

    /*
     * Index of the segment that precedes this one in the same segment bin, or
     * DSA_SEGMENT_INDEX_NONE if this is the first one.
     */
    dsa_segment_index prev;

    /*
     * Index of the segment that follows this one in the same segment bin, or
     * DSA_SEGMENT_INDEX_NONE if this is the last one.
     */
    dsa_segment_index next;
    /* The index of the bin that contains this segment. */
    size_t      bin;

    /*
     * A flag raised to indicate that this segment is being returned to the
     * operating system and has been unpinned.
     */
    bool        freed;
} dsa_segment_header;

/*
 * Metadata for one superblock.
 *
 * For most blocks, span objects are stored out-of-line; that is, the span
 * object is not stored within the block itself.  But, as an exception, for a
 * "span of spans", the span object is stored "inline".  The allocation is
 * always exactly one page, and the dsa_area_span object is located at
 * the beginning of that page.  The size class is DSA_SCLASS_BLOCK_OF_SPANS,
 * and the remaining fields are used just as they would be in an ordinary
 * block.  We can't allocate spans out of ordinary superblocks because
 * creating an ordinary superblock requires us to be able to allocate a span
 * *first*.  Doing it this way avoids that circularity.
 */
typedef struct
{
    dsa_pointer pool;           /* Containing pool. */
    dsa_pointer prevspan;       /* Previous span. */
    dsa_pointer nextspan;       /* Next span. */
    dsa_pointer start;          /* Starting address. */
    size_t      npages;         /* Length of span in pages. */
    uint16      size_class;     /* Size class. */
    uint16      ninitialized;   /* Maximum number of objects ever allocated. */
    uint16      nallocatable;   /* Number of objects currently allocatable. */
    uint16      firstfree;      /* First object on free list. */
    uint16      nmax;           /* Maximum number of objects ever possible. */
    uint16      fclass;         /* Current fullness class. */
} dsa_area_span;

/*
 * Given a pointer to an object in a span, access the index of the next free
 * object in the same span (ie in the span's freelist) as an L-value.
 */
#define NextFreeObjectIndex(object) (* (uint16 *) (object))

/*
 * Small allocations are handled by dividing a single block of memory into
 * many small objects of equal size.  The possible allocation sizes are
 * defined by the following array.  Larger size classes are spaced more widely
 * than smaller size classes.  We fudge the spacing for size classes >1kB to
 * avoid space wastage: based on the knowledge that we plan to allocate 64kB
 * blocks, we bump the maximum object size up to the largest multiple of
 * 8 bytes that still lets us fit the same number of objects into one block.
 *
 * NB: Because of this fudging, if we were ever to use differently-sized blocks
 * for small allocations, these size classes would need to be reworked to be
 * optimal for the new size.
 *
 * NB: The optimal spacing for size classes, as well as the size of the blocks
 * out of which small objects are allocated, is not a question that has one
 * right answer.  Some allocators (such as tcmalloc) use more closely-spaced
 * size classes than we do here, while others (like aset.c) use more
 * widely-spaced classes.  Spacing the classes more closely avoids wasting
 * memory within individual chunks, but also means a larger number of
 * potentially-unfilled blocks.
 */
static const uint16 dsa_size_classes[] = {
    sizeof(dsa_area_span), 0,   /* special size classes */
    8, 16, 24, 32, 40, 48, 56, 64,  /* 8 classes separated by 8 bytes */
    80, 96, 112, 128,           /* 4 classes separated by 16 bytes */
    160, 192, 224, 256,         /* 4 classes separated by 32 bytes */
    320, 384, 448, 512,         /* 4 classes separated by 64 bytes */
    640, 768, 896, 1024,        /* 4 classes separated by 128 bytes */
    1280, 1560, 1816, 2048,     /* 4 classes separated by ~256 bytes */
    2616, 3120, 3640, 4096,     /* 4 classes separated by ~512 bytes */
    5456, 6552, 7280, 8192      /* 4 classes separated by ~1024 bytes */
};
#define DSA_NUM_SIZE_CLASSES                lengthof(dsa_size_classes)

/* Special size classes. */
#define DSA_SCLASS_BLOCK_OF_SPANS       0
#define DSA_SCLASS_SPAN_LARGE           1

/*
 * The following lookup table is used to map the size of small objects
 * (less than 1kB) onto the corresponding size class.  To use this table,
 * round the size of the object up to the next multiple of 8 bytes, and then
 * index into this array.
 */
static const uint8 dsa_size_class_map[] = {
    2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 11, 11, 12, 12, 13, 13,
    14, 14, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 17, 17, 17, 17,
    18, 18, 18, 18, 18, 18, 18, 18, 19, 19, 19, 19, 19, 19, 19, 19,
    20, 20, 20, 20, 20, 20, 20, 20, 21, 21, 21, 21, 21, 21, 21, 21,
    22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
    23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25
};
#define DSA_SIZE_CLASS_MAP_QUANTUM  8

/*
 * Superblocks are binned by how full they are.  Generally, each fullness
 * class corresponds to one quartile, but the block being used for
 * allocations is always at the head of the list for fullness class 1,
 * regardless of how full it really is.
 */
#define DSA_FULLNESS_CLASSES        4

/*
 * A dsa_area_pool represents a set of objects of a given size class.
 *
 * Perhaps there should be multiple pools for the same size class for
 * contention avoidance, but for now there is just one!
 */
typedef struct
{
    /* A lock protecting access to this pool. */
    LWLock      lock;
    /* A set of linked lists of spans, arranged by fullness. */
    dsa_pointer spans[DSA_FULLNESS_CLASSES];
    /* Should we pad this out to a cacheline boundary? */
} dsa_area_pool;

/*
 * The control block for an area.  This lives in shared memory, at the start of
 * the first DSM segment controlled by this area.
 */
typedef struct
{
    /* The segment header for the first segment. */
    dsa_segment_header segment_header;
    /* The handle for this area. */
    dsa_handle  handle;
    /* The handles of the segments owned by this area. */
    dsm_handle  segment_handles[DSA_MAX_SEGMENTS];
    /* Lists of segments, binned by maximum contiguous run of free pages. */
    dsa_segment_index segment_bins[DSA_NUM_SEGMENT_BINS];
    /* The object pools for each size class. */
    dsa_area_pool pools[DSA_NUM_SIZE_CLASSES];
    /* The total size of all active segments. */
    size_t      total_segment_size;
    /* The maximum total size of backing storage we are allowed. */
    size_t      max_total_segment_size;
    /* Highest used segment index in the history of this area. */
    dsa_segment_index high_segment_index;
    /* The reference count for this area. */
    int         refcnt;
    /* A flag indicating that this area has been pinned. */
    bool        pinned;
    /* The number of times that segments have been freed. */
    size_t      freed_segment_counter;
    /* The LWLock tranche ID. */
    int         lwlock_tranche_id;
    /* The general lock (protects everything except object pools). */
    LWLock      lock;
} dsa_area_control;

/* Given a pointer to a pool, find a dsa_pointer. */
#define DsaAreaPoolToDsaPointer(area, p)    \
    DSA_MAKE_POINTER(0, (char *) p - (char *) area->control)

/*
 * A dsa_segment_map is stored within the backend-private memory of each
 * individual backend.  It holds the base address of the segment within that
 * backend, plus the addresses of key objects within the segment.  Those
 * could instead be derived from the base address but it's handy to have them
 * around.
 */
typedef struct
{
    dsm_segment *segment;       /* DSM segment */
    char       *mapped_address; /* Address at which segment is mapped */
    dsa_segment_header *header; /* Header (same as mapped_address) */
    FreePageManager *fpm;       /* Free page manager within segment. */
    dsa_pointer *pagemap;       /* Page map within segment. */
} dsa_segment_map;

/*
 * Per-backend state for a storage area.  Backends obtain one of these by
 * creating an area or attaching to an existing one using a handle.  Each
 * process that needs to use an area uses its own object to track where the
 * segments are mapped.
 */
struct dsa_area
{
    /* Pointer to the control object in shared memory. */
    dsa_area_control *control;

    /* Has the mapping been pinned? */
    bool        mapping_pinned;

    /*
     * This backend's array of segment maps, ordered by segment index
     * corresponding to control->segment_handles.  Some of the area's segments
     * may not be mapped in this backend yet, and some slots may have been
     * freed and need to be detached; these operations happen on demand.
     */
    dsa_segment_map segment_maps[DSA_MAX_SEGMENTS];

    /* The highest segment index this backend has ever mapped. */
    dsa_segment_index high_segment_index;

    /* The last observed freed_segment_counter. */
    size_t      freed_segment_counter;
};

#define DSA_SPAN_NOTHING_FREE   ((uint16) -1)
#define DSA_SUPERBLOCK_SIZE (DSA_PAGES_PER_SUPERBLOCK * FPM_PAGE_SIZE)

/* Given a pointer to a segment_map, obtain a segment index number. */
#define get_segment_index(area, segment_map_ptr) \
    (segment_map_ptr - &area->segment_maps[0])

static void init_span(dsa_area *area, dsa_pointer span_pointer,
                      dsa_area_pool *pool, dsa_pointer start, size_t npages,
                      uint16 size_class);
static bool transfer_first_span(dsa_area *area, dsa_area_pool *pool,
                                int fromclass, int toclass);
static inline dsa_pointer alloc_object(dsa_area *area, int size_class);
static bool ensure_active_superblock(dsa_area *area, dsa_area_pool *pool,
                                     int size_class);
static dsa_segment_map *get_segment_by_index(dsa_area *area,
                                             dsa_segment_index index);
static void destroy_superblock(dsa_area *area, dsa_pointer span_pointer);
static void unlink_span(dsa_area *area, dsa_area_span *span);
static void add_span_to_fullness_class(dsa_area *area, dsa_area_span *span,
                                       dsa_pointer span_pointer, int fclass);
static void unlink_segment(dsa_area *area, dsa_segment_map *segment_map);
static dsa_segment_map *get_best_segment(dsa_area *area, size_t npages);
static dsa_segment_map *make_new_segment(dsa_area *area, size_t requested_pages);
static dsa_area *create_internal(void *place, size_t size,
                                 int tranche_id,
                                 dsm_handle control_handle,
                                 dsm_segment *control_segment);
static dsa_area *attach_internal(void *place, dsm_segment *segment,
                                 dsa_handle handle);
static void check_for_freed_segments(dsa_area *area);
static void check_for_freed_segments_locked(dsa_area *area);

/*
 * Create a new shared area in a new DSM segment.  Further DSM segments will
 * be allocated as required to extend the available space.
 *
 * We can't allocate a LWLock tranche_id within this function, because tranche
 * IDs are a scarce resource; there are only 64k available, using low numbers
 * when possible matters, and we have no provision for recycling them.  So,
 * we require the caller to provide one.
 */
dsa_area *
dsa_create(int tranche_id)
{
    dsm_segment *segment;
    dsa_area   *area;

    /*
     * Create the DSM segment that will hold the shared control object and the
     * first segment of usable space.
     */
    segment = dsm_create(DSA_INITIAL_SEGMENT_SIZE, 0);

    /*
     * All segments backing this area are pinned, so that DSA can explicitly
     * control their lifetime (otherwise a newly created segment belonging to
     * this area might be freed when the only backend that happens to have it
     * mapped in ends, corrupting the area).
     */
    dsm_pin_segment(segment);

    /* Create a new DSA area with the control object in this segment. */
    area = create_internal(dsm_segment_address(segment),
                           DSA_INITIAL_SEGMENT_SIZE,
                           tranche_id,
                           dsm_segment_handle(segment), segment);

    /* Clean up when the control segment detaches. */
    on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                  PointerGetDatum(dsm_segment_address(segment)));

    return area;
}

/*
 * Create a new shared area in an existing shared memory space, which may be
 * either DSM or Postmaster-initialized memory.  DSM segments will be
 * allocated as required to extend the available space, though that can be
 * prevented with dsa_set_size_limit(area, size) using the same size provided
 * to dsa_create_in_place.
 *
 * Areas created in-place must eventually be released by the backend that
 * created them and all backends that attach to them.  This can be done
 * explicitly with dsa_release_in_place, or, in the special case that 'place'
 * happens to be in a pre-existing DSM segment, by passing in a pointer to the
 * segment so that a detach hook can be registered with the containing DSM
 * segment.
 *
 * See dsa_create() for a note about the tranche arguments.
 */
dsa_area *
dsa_create_in_place(void *place, size_t size,
                    int tranche_id, dsm_segment *segment)
{
    dsa_area   *area;

    area = create_internal(place, size, tranche_id,
                           DSM_HANDLE_INVALID, NULL);

    /*
     * Clean up when the control segment detaches, if a containing DSM segment
     * was provided.
     */
    if (segment != NULL)
        on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                      PointerGetDatum(place));

    return area;
}

/*
 * Obtain a handle that can be passed to other processes so that they can
 * attach to the given area.  Cannot be called for areas created with
 * dsa_create_in_place.
 */
dsa_handle
dsa_get_handle(dsa_area *area)
{
    Assert(area->control->handle != DSM_HANDLE_INVALID);
    return area->control->handle;
}

/*
 * Attach to an area given a handle generated (possibly in another process) by
 * dsa_get_handle.  The area must have been created with dsa_create (not
 * dsa_create_in_place).
 */
dsa_area *
dsa_attach(dsa_handle handle)
{
    dsm_segment *segment;
    dsa_area   *area;

    /*
     * An area handle is really a DSM segment handle for the first segment, so
     * we go ahead and attach to that.
     */
    segment = dsm_attach(handle);
    if (segment == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("could not attach to dynamic shared area")));

    area = attach_internal(dsm_segment_address(segment), segment, handle);

    /* Clean up when the control segment detaches. */
    on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                  PointerGetDatum(dsm_segment_address(segment)));

    return area;
}

/*
 * Attach to an area that was created with dsa_create_in_place.  The caller
 * must somehow know the location in memory that was used when the area was
 * created, though it may be mapped at a different virtual address in this
 * process.
 *
 * See dsa_create_in_place for note about releasing in-place areas, and the
 * optional 'segment' argument which can be provided to allow automatic
 * release if the containing memory happens to be a DSM segment.
 */
dsa_area *
dsa_attach_in_place(void *place, dsm_segment *segment)
{
    dsa_area   *area;

    area = attach_internal(place, NULL, DSM_HANDLE_INVALID);

    /*
     * Clean up when the control segment detaches, if a containing DSM segment
     * was provided.
     */
    if (segment != NULL)
        on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,
                      PointerGetDatum(place));

    return area;
}

/*
 * Release a DSA area that was produced by dsa_create_in_place or
 * dsa_attach_in_place.  The 'segment' argument is ignored but provides an
 * interface suitable for on_dsm_detach, for the convenience of users who want
 * to create a DSA segment inside an existing DSM segment and have it
 * automatically released when the containing DSM segment is detached.
 * 'place' should be the address of the place where the area was created.
 *
 * This callback is automatically registered for the DSM segment containing
 * the control object of in-place areas when a segment is provided to
 * dsa_create_in_place or dsa_attach_in_place, and also for all areas created
 * with dsa_create.
 */
void
dsa_on_dsm_detach_release_in_place(dsm_segment *segment, Datum place)
{
    dsa_release_in_place(DatumGetPointer(place));
}

/*
 * Release a DSA area that was produced by dsa_create_in_place or
 * dsa_attach_in_place.  The 'code' argument is ignored but provides an
 * interface suitable for on_shmem_exit or before_shmem_exit, for the
 * convenience of users who want to create a DSA segment inside shared memory
 * other than a DSM segment and have it automatically release at backend exit.
 * 'place' should be the address of the place where the area was created.
 */
void
dsa_on_shmem_exit_release_in_place(int code, Datum place)
{
    dsa_release_in_place(DatumGetPointer(place));
}

/*
 * Release a DSA area that was produced by dsa_create_in_place or
 * dsa_attach_in_place.  It is preferable to use one of the 'dsa_on_XXX'
 * callbacks so that this is managed automatically, because failure to release
 * an area created in-place leaks its segments permanently.
 *
 * This is also called automatically for areas produced by dsa_create or
 * dsa_attach as an implementation detail.
 */
void
dsa_release_in_place(void *place)
{
    dsa_area_control *control = (dsa_area_control *) place;
    int         i;

    LWLockAcquire(&control->lock, LW_EXCLUSIVE);
    Assert(control->segment_header.magic ==
           (DSA_SEGMENT_HEADER_MAGIC ^ control->handle ^ 0));
    Assert(control->refcnt > 0);
    if (--control->refcnt == 0)
    {
        for (i = 0; i <= control->high_segment_index; ++i)
        {
            dsm_handle  handle;

            handle = control->segment_handles[i];
            if (handle != DSM_HANDLE_INVALID)
                dsm_unpin_segment(handle);
        }
    }
    LWLockRelease(&control->lock);
}

/*
 * Keep a DSA area attached until end of session or explicit detach.
 *
 * By default, areas are owned by the current resource owner, which means they
 * are detached automatically when that scope ends.
 */
void
dsa_pin_mapping(dsa_area *area)
{
    int         i;

    Assert(!area->mapping_pinned);
    area->mapping_pinned = true;

    for (i = 0; i <= area->high_segment_index; ++i)
        if (area->segment_maps[i].segment != NULL)
            dsm_pin_mapping(area->segment_maps[i].segment);
}

/*
 * Allocate memory in this storage area.  The return value is a dsa_pointer
 * that can be passed to other processes, and converted to a local pointer
 * with dsa_get_address.  'flags' is a bitmap which should be constructed
 * from the following values:
 *
 * DSA_ALLOC_HUGE allows allocations >= 1GB.  Otherwise, such allocations
 * will result in an ERROR.
 *
 * DSA_ALLOC_NO_OOM causes this function to return InvalidDsaPointer when
 * no memory is available or a size limit established by dsa_set_size_limit
 * would be exceeded.  Otherwise, such allocations will result in an ERROR.
 *
 * DSA_ALLOC_ZERO causes the allocated memory to be zeroed.  Otherwise, the
 * contents of newly-allocated memory are indeterminate.
 *
 * These flags correspond to similarly named flags used by
 * MemoryContextAllocExtended().  See also the macros dsa_allocate and
 * dsa_allocate0 which expand to a call to this function with commonly used
 * flags.
 */
dsa_pointer
dsa_allocate_extended(dsa_area *area, size_t size, int flags)
{
    uint16      size_class;
    dsa_pointer start_pointer;
    dsa_segment_map *segment_map;
    dsa_pointer result;

    Assert(size > 0);

    /* Sanity check on huge individual allocation size. */
    if (((flags & DSA_ALLOC_HUGE) != 0 && !AllocHugeSizeIsValid(size)) ||
        ((flags & DSA_ALLOC_HUGE) == 0 && !AllocSizeIsValid(size)))
        elog(ERROR, "invalid DSA memory alloc request size %zu", size);

    /*
     * If bigger than the largest size class, just grab a run of pages from
     * the free page manager, instead of allocating an object from a pool.
     * There will still be a span, but it's a special class of span that
     * manages this whole allocation and simply gives all pages back to the
     * free page manager when dsa_free is called.
     */
    if (size > dsa_size_classes[lengthof(dsa_size_classes) - 1])
    {
        size_t      npages = fpm_size_to_pages(size);
        size_t      first_page;
        dsa_pointer span_pointer;
        dsa_area_pool *pool = &area->control->pools[DSA_SCLASS_SPAN_LARGE];

        /* Obtain a span object. */
        span_pointer = alloc_object(area, DSA_SCLASS_BLOCK_OF_SPANS);
        if (!DsaPointerIsValid(span_pointer))
        {
            /* Raise error unless asked not to. */
            if ((flags & DSA_ALLOC_NO_OOM) == 0)
                ereport(ERROR,
                        (errcode(ERRCODE_OUT_OF_MEMORY),
                         errmsg("out of memory"),
                         errdetail("Failed on DSA request of size %zu.",
                                   size)));
            return InvalidDsaPointer;
        }

        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);

        /* Find a segment from which to allocate. */
        segment_map = get_best_segment(area, npages);
        if (segment_map == NULL)
            segment_map = make_new_segment(area, npages);
        if (segment_map == NULL)
        {
            /* Can't make any more segments: game over. */
            LWLockRelease(DSA_AREA_LOCK(area));
            dsa_free(area, span_pointer);

            /* Raise error unless asked not to. */
            if ((flags & DSA_ALLOC_NO_OOM) == 0)
                ereport(ERROR,
                        (errcode(ERRCODE_OUT_OF_MEMORY),
                         errmsg("out of memory"),
                         errdetail("Failed on DSA request of size %zu.",
                                   size)));
            return InvalidDsaPointer;
        }

        /*
         * Ask the free page manager for a run of pages.  This should always
         * succeed, since both get_best_segment and make_new_segment should
         * only return a non-NULL pointer if it actually contains enough
         * contiguous freespace.  If it does fail, something in our backend
         * private state is out of whack, so use FATAL to kill the process.
         */
        if (!FreePageManagerGet(segment_map->fpm, npages, &first_page))
            elog(FATAL,
                 "dsa_allocate could not find %zu free pages", npages);
        LWLockRelease(DSA_AREA_LOCK(area));

        start_pointer = DSA_MAKE_POINTER(get_segment_index(area, segment_map),
                                         first_page * FPM_PAGE_SIZE);

        /* Initialize span and pagemap. */
        LWLockAcquire(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE),
                      LW_EXCLUSIVE);
        init_span(area, span_pointer, pool, start_pointer, npages,
                  DSA_SCLASS_SPAN_LARGE);
        segment_map->pagemap[first_page] = span_pointer;
        LWLockRelease(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE));

        /* Zero-initialize the memory if requested. */
        if ((flags & DSA_ALLOC_ZERO) != 0)
            memset(dsa_get_address(area, start_pointer), 0, size);

        return start_pointer;
    }

    /* Map allocation to a size class. */
    if (size < lengthof(dsa_size_class_map) * DSA_SIZE_CLASS_MAP_QUANTUM)
    {
        int         mapidx;

        /* For smaller sizes we have a lookup table... */
        mapidx = ((size + DSA_SIZE_CLASS_MAP_QUANTUM - 1) /
                  DSA_SIZE_CLASS_MAP_QUANTUM) - 1;
        size_class = dsa_size_class_map[mapidx];
    }
    else
    {
        uint16      min;
        uint16      max;

        /* ... and for the rest we search by binary chop. */
        min = dsa_size_class_map[lengthof(dsa_size_class_map) - 1];
        max = lengthof(dsa_size_classes) - 1;

        while (min < max)
        {
            uint16      mid = (min + max) / 2;
            uint16      class_size = dsa_size_classes[mid];

            if (class_size < size)
                min = mid + 1;
            else
                max = mid;
        }

        size_class = min;
    }
    Assert(size <= dsa_size_classes[size_class]);
    Assert(size_class == 0 || size > dsa_size_classes[size_class - 1]);

    /* Attempt to allocate an object from the appropriate pool. */
    result = alloc_object(area, size_class);

    /* Check for failure to allocate. */
    if (!DsaPointerIsValid(result))
    {
        /* Raise error unless asked not to. */
        if ((flags & DSA_ALLOC_NO_OOM) == 0)
            ereport(ERROR,
                    (errcode(ERRCODE_OUT_OF_MEMORY),
                     errmsg("out of memory"),
                     errdetail("Failed on DSA request of size %zu.", size)));
        return InvalidDsaPointer;
    }

    /* Zero-initialize the memory if requested. */
    if ((flags & DSA_ALLOC_ZERO) != 0)
        memset(dsa_get_address(area, result), 0, size);

    return result;
}

/*
 * Free memory obtained with dsa_allocate.
 */
void
dsa_free(dsa_area *area, dsa_pointer dp)
{
    dsa_segment_map *segment_map;
    int         pageno;
    dsa_pointer span_pointer;
    dsa_area_span *span;
    char       *superblock;
    char       *object;
    size_t      size;
    int         size_class;

    /* Make sure we don't have a stale segment in the slot 'dp' refers to. */
    check_for_freed_segments(area);

    /* Locate the object, span and pool. */
    segment_map = get_segment_by_index(area, DSA_EXTRACT_SEGMENT_NUMBER(dp));
    pageno = DSA_EXTRACT_OFFSET(dp) / FPM_PAGE_SIZE;
    span_pointer = segment_map->pagemap[pageno];
    span = dsa_get_address(area, span_pointer);
    superblock = dsa_get_address(area, span->start);
    object = dsa_get_address(area, dp);
    size_class = span->size_class;
    size = dsa_size_classes[size_class];

    /*
     * Special case for large objects that live in a special span: we return
     * those pages directly to the free page manager and free the span.
     */
    if (span->size_class == DSA_SCLASS_SPAN_LARGE)
    {

#ifdef CLOBBER_FREED_MEMORY
        memset(object, 0x7f, span->npages * FPM_PAGE_SIZE);
#endif

        /* Give pages back to free page manager. */
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        FreePageManagerPut(segment_map->fpm,
                           DSA_EXTRACT_OFFSET(span->start) / FPM_PAGE_SIZE,
                           span->npages);
        LWLockRelease(DSA_AREA_LOCK(area));
        /* Unlink span. */
        LWLockAcquire(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE),
                      LW_EXCLUSIVE);
        unlink_span(area, span);
        LWLockRelease(DSA_SCLASS_LOCK(area, DSA_SCLASS_SPAN_LARGE));
        /* Free the span object so it can be reused. */
        dsa_free(area, span_pointer);
        return;
    }

#ifdef CLOBBER_FREED_MEMORY
    memset(object, 0x7f, size);
#endif

    LWLockAcquire(DSA_SCLASS_LOCK(area, size_class), LW_EXCLUSIVE);

    /* Put the object on the span's freelist. */
    Assert(object >= superblock);
    Assert(object < superblock + DSA_SUPERBLOCK_SIZE);
    Assert((object - superblock) % size == 0);
    NextFreeObjectIndex(object) = span->firstfree;
    span->firstfree = (object - superblock) / size;
    ++span->nallocatable;

    /*
     * See if the span needs to moved to a different fullness class, or be
     * freed so its pages can be given back to the segment.
     */
    if (span->nallocatable == 1 && span->fclass == DSA_FULLNESS_CLASSES - 1)
    {
        /*
         * The block was completely full and is located in the
         * highest-numbered fullness class, which is never scanned for free
         * chunks.  We must move it to the next-lower fullness class.
         */
        unlink_span(area, span);
        add_span_to_fullness_class(area, span, span_pointer,
                                   DSA_FULLNESS_CLASSES - 2);

        /*
         * If this is the only span, and there is no active span, then we
         * should probably move this span to fullness class 1.  (Otherwise if
         * you allocate exactly all the objects in the only span, it moves to
         * class 3, then you free them all, it moves to 2, and then is given
         * back, leaving no active span).
         */
    }
    else if (span->nallocatable == span->nmax &&
             (span->fclass != 1 || span->prevspan != InvalidDsaPointer))
    {
        /*
         * This entire block is free, and it's not the active block for this
         * size class.  Return the memory to the free page manager. We don't
         * do this for the active block to prevent hysteresis: if we
         * repeatedly allocate and free the only chunk in the active block, it
         * will be very inefficient if we deallocate and reallocate the block
         * every time.
         */
        destroy_superblock(area, span_pointer);
    }

    LWLockRelease(DSA_SCLASS_LOCK(area, size_class));
}

/*
 * Obtain a backend-local address for a dsa_pointer.  'dp' must point to
 * memory allocated by the given area (possibly in another process) that
 * hasn't yet been freed.  This may cause a segment to be mapped into the
 * current process if required, and may cause freed segments to be unmapped.
 */
void *
dsa_get_address(dsa_area *area, dsa_pointer dp)
{
    dsa_segment_index index;
    size_t      offset;

    /* Convert InvalidDsaPointer to NULL. */
    if (!DsaPointerIsValid(dp))
        return NULL;

    /* Process any requests to detach from freed segments. */
    check_for_freed_segments(area);

    /* Break the dsa_pointer into its components. */
    index = DSA_EXTRACT_SEGMENT_NUMBER(dp);
    offset = DSA_EXTRACT_OFFSET(dp);
    Assert(index < DSA_MAX_SEGMENTS);

    /* Check if we need to cause this segment to be mapped in. */
    if (unlikely(area->segment_maps[index].mapped_address == NULL))
    {
        /* Call for effect (we don't need the result). */
        get_segment_by_index(area, index);
    }

    return area->segment_maps[index].mapped_address + offset;
}

/*
 * Pin this area, so that it will continue to exist even if all backends
 * detach from it.  In that case, the area can still be reattached to if a
 * handle has been recorded somewhere.
 */
void
dsa_pin(dsa_area *area)
{
    LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
    if (area->control->pinned)
    {
        LWLockRelease(DSA_AREA_LOCK(area));
        elog(ERROR, "dsa_area already pinned");
    }
    area->control->pinned = true;
    ++area->control->refcnt;
    LWLockRelease(DSA_AREA_LOCK(area));
}

/*
 * Undo the effects of dsa_pin, so that the given area can be freed when no
 * backends are attached to it.  May be called only if dsa_pin has been
 * called.
 */
void
dsa_unpin(dsa_area *area)
{
    LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
    Assert(area->control->refcnt > 1);
    if (!area->control->pinned)
    {
        LWLockRelease(DSA_AREA_LOCK(area));
        elog(ERROR, "dsa_area not pinned");
    }
    area->control->pinned = false;
    --area->control->refcnt;
    LWLockRelease(DSA_AREA_LOCK(area));
}

/*
 * Set the total size limit for this area.  This limit is checked whenever new
 * segments need to be allocated from the operating system.  If the new size
 * limit is already exceeded, this has no immediate effect.
 *
 * Note that the total virtual memory usage may be temporarily larger than
 * this limit when segments have been freed, but not yet detached by all
 * backends that have attached to them.
 */
void
dsa_set_size_limit(dsa_area *area, size_t limit)
{
    LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
    area->control->max_total_segment_size = limit;
    LWLockRelease(DSA_AREA_LOCK(area));
}

/*
 * Aggressively free all spare memory in the hope of returning DSM segments to
 * the operating system.
 */
void
dsa_trim(dsa_area *area)
{
    int         size_class;

    /*
     * Trim in reverse pool order so we get to the spans-of-spans last, just
     * in case any become entirely free while processing all the other pools.
     */
    for (size_class = DSA_NUM_SIZE_CLASSES - 1; size_class >= 0; --size_class)
    {
        dsa_area_pool *pool = &area->control->pools[size_class];
        dsa_pointer span_pointer;

        if (size_class == DSA_SCLASS_SPAN_LARGE)
        {
            /* Large object frees give back segments aggressively already. */
            continue;
        }

        /*
         * Search fullness class 1 only.  That is where we expect to find an
         * entirely empty superblock (entirely empty superblocks in other
         * fullness classes are returned to the free page map by dsa_free).
         */
        LWLockAcquire(DSA_SCLASS_LOCK(area, size_class), LW_EXCLUSIVE);
        span_pointer = pool->spans[1];
        while (DsaPointerIsValid(span_pointer))
        {
            dsa_area_span *span = dsa_get_address(area, span_pointer);
            dsa_pointer next = span->nextspan;

            if (span->nallocatable == span->nmax)
                destroy_superblock(area, span_pointer);

            span_pointer = next;
        }
        LWLockRelease(DSA_SCLASS_LOCK(area, size_class));
    }
}

/*
 * Print out debugging information about the internal state of the shared
 * memory area.
 */
void
dsa_dump(dsa_area *area)
{
    size_t      i,
                j;

    /*
     * Note: This gives an inconsistent snapshot as it acquires and releases
     * individual locks as it goes...
     */

    LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
    check_for_freed_segments_locked(area);
    fprintf(stderr, "dsa_area handle %x:\n", area->control->handle);
    fprintf(stderr, "  max_total_segment_size: %zu\n",
            area->control->max_total_segment_size);
    fprintf(stderr, "  total_segment_size: %zu\n",
            area->control->total_segment_size);
    fprintf(stderr, "  refcnt: %d\n", area->control->refcnt);
    fprintf(stderr, "  pinned: %c\n", area->control->pinned ? 't' : 'f');
    fprintf(stderr, "  segment bins:\n");
    for (i = 0; i < DSA_NUM_SEGMENT_BINS; ++i)
    {
        if (area->control->segment_bins[i] != DSA_SEGMENT_INDEX_NONE)
        {
            dsa_segment_index segment_index;

            fprintf(stderr,
                    "    segment bin %zu (at least %d contiguous pages free):\n",
                    i, 1 << (i - 1));
            segment_index = area->control->segment_bins[i];
            while (segment_index != DSA_SEGMENT_INDEX_NONE)
            {
                dsa_segment_map *segment_map;

                segment_map =
                    get_segment_by_index(area, segment_index);

                fprintf(stderr,
                        "      segment index %zu, usable_pages = %zu, "
                        "contiguous_pages = %zu, mapped at %p\n",
                        segment_index,
                        segment_map->header->usable_pages,
                        fpm_largest(segment_map->fpm),
                        segment_map->mapped_address);
                segment_index = segment_map->header->next;
            }
        }
    }
    LWLockRelease(DSA_AREA_LOCK(area));

    fprintf(stderr, "  pools:\n");
    for (i = 0; i < DSA_NUM_SIZE_CLASSES; ++i)
    {
        bool        found = false;

        LWLockAcquire(DSA_SCLASS_LOCK(area, i), LW_EXCLUSIVE);
        for (j = 0; j < DSA_FULLNESS_CLASSES; ++j)
            if (DsaPointerIsValid(area->control->pools[i].spans[j]))
                found = true;
        if (found)
        {
            if (i == DSA_SCLASS_BLOCK_OF_SPANS)
                fprintf(stderr, "    pool for blocks of span objects:\n");
            else if (i == DSA_SCLASS_SPAN_LARGE)
                fprintf(stderr, "    pool for large object spans:\n");
            else
                fprintf(stderr,
                        "    pool for size class %zu (object size %hu bytes):\n",
                        i, dsa_size_classes[i]);
            for (j = 0; j < DSA_FULLNESS_CLASSES; ++j)
            {
                if (!DsaPointerIsValid(area->control->pools[i].spans[j]))
                    fprintf(stderr, "      fullness class %zu is empty\n", j);
                else
                {
                    dsa_pointer span_pointer = area->control->pools[i].spans[j];

                    fprintf(stderr, "      fullness class %zu:\n", j);
                    while (DsaPointerIsValid(span_pointer))
                    {
                        dsa_area_span *span;

                        span = dsa_get_address(area, span_pointer);
                        fprintf(stderr,
                                "        span descriptor at "
                                DSA_POINTER_FORMAT ", superblock at "
                                DSA_POINTER_FORMAT
                                ", pages = %zu, objects free = %hu/%hu\n",
                                span_pointer, span->start, span->npages,
                                span->nallocatable, span->nmax);
                        span_pointer = span->nextspan;
                    }
                }
            }
        }
        LWLockRelease(DSA_SCLASS_LOCK(area, i));
    }
}

/*
 * Return the smallest size that you can successfully provide to
 * dsa_create_in_place.
 */
size_t
dsa_minimum_size(void)
{
    size_t      size;
    int         pages = 0;

    size = MAXALIGN(sizeof(dsa_area_control)) +
        MAXALIGN(sizeof(FreePageManager));

    /* Figure out how many pages we need, including the page map... */
    while (((size + FPM_PAGE_SIZE - 1) / FPM_PAGE_SIZE) > pages)
    {
        ++pages;
        size += sizeof(dsa_pointer);
    }

    return pages * FPM_PAGE_SIZE;
}

/*
 * Workhorse function for dsa_create and dsa_create_in_place.
 */
static dsa_area *
create_internal(void *place, size_t size,
                int tranche_id,
                dsm_handle control_handle,
                dsm_segment *control_segment)
{
    dsa_area_control *control;
    dsa_area   *area;
    dsa_segment_map *segment_map;
    size_t      usable_pages;
    size_t      total_pages;
    size_t      metadata_bytes;
    int         i;

    /* Sanity check on the space we have to work in. */
    if (size < dsa_minimum_size())
        elog(ERROR, "dsa_area space must be at least %zu, but %zu provided",
             dsa_minimum_size(), size);

    /* Now figure out how much space is usable */
    total_pages = size / FPM_PAGE_SIZE;
    metadata_bytes =
        MAXALIGN(sizeof(dsa_area_control)) +
        MAXALIGN(sizeof(FreePageManager)) +
        total_pages * sizeof(dsa_pointer);
    /* Add padding up to next page boundary. */
    if (metadata_bytes % FPM_PAGE_SIZE != 0)
        metadata_bytes += FPM_PAGE_SIZE - (metadata_bytes % FPM_PAGE_SIZE);
    Assert(metadata_bytes <= size);
    usable_pages = (size - metadata_bytes) / FPM_PAGE_SIZE;

    /*
     * Initialize the dsa_area_control object located at the start of the
     * space.
     */
    control = (dsa_area_control *) place;
    control->segment_header.magic =
        DSA_SEGMENT_HEADER_MAGIC ^ control_handle ^ 0;
    control->segment_header.next = DSA_SEGMENT_INDEX_NONE;
    control->segment_header.prev = DSA_SEGMENT_INDEX_NONE;
    control->segment_header.usable_pages = usable_pages;
    control->segment_header.freed = false;
    control->segment_header.size = DSA_INITIAL_SEGMENT_SIZE;
    control->handle = control_handle;
    control->max_total_segment_size = (size_t) -1;
    control->total_segment_size = size;
    memset(&control->segment_handles[0], 0,
           sizeof(dsm_handle) * DSA_MAX_SEGMENTS);
    control->segment_handles[0] = control_handle;
    for (i = 0; i < DSA_NUM_SEGMENT_BINS; ++i)
        control->segment_bins[i] = DSA_SEGMENT_INDEX_NONE;
    control->high_segment_index = 0;
    control->refcnt = 1;
    control->freed_segment_counter = 0;
    control->lwlock_tranche_id = tranche_id;

    /*
     * Create the dsa_area object that this backend will use to access the
     * area.  Other backends will need to obtain their own dsa_area object by
     * attaching.
     */
    area = palloc(sizeof(dsa_area));
    area->control = control;
    area->mapping_pinned = false;
    memset(area->segment_maps, 0, sizeof(dsa_segment_map) * DSA_MAX_SEGMENTS);
    area->high_segment_index = 0;
    area->freed_segment_counter = 0;
    LWLockInitialize(&control->lock, control->lwlock_tranche_id);
    for (i = 0; i < DSA_NUM_SIZE_CLASSES; ++i)
        LWLockInitialize(DSA_SCLASS_LOCK(area, i),
                         control->lwlock_tranche_id);

    /* Set up the segment map for this process's mapping. */
    segment_map = &area->segment_maps[0];
    segment_map->segment = control_segment;
    segment_map->mapped_address = place;
    segment_map->header = (dsa_segment_header *) place;
    segment_map->fpm = (FreePageManager *)
        (segment_map->mapped_address +
         MAXALIGN(sizeof(dsa_area_control)));
    segment_map->pagemap = (dsa_pointer *)
        (segment_map->mapped_address +
         MAXALIGN(sizeof(dsa_area_control)) +
         MAXALIGN(sizeof(FreePageManager)));

    /* Set up the free page map. */
    FreePageManagerInitialize(segment_map->fpm, segment_map->mapped_address);
    /* There can be 0 usable pages if size is dsa_minimum_size(). */

    if (usable_pages > 0)
        FreePageManagerPut(segment_map->fpm, metadata_bytes / FPM_PAGE_SIZE,
                           usable_pages);

    /* Put this segment into the appropriate bin. */
    control->segment_bins[contiguous_pages_to_segment_bin(usable_pages)] = 0;
    segment_map->header->bin = contiguous_pages_to_segment_bin(usable_pages);

    return area;
}

/*
 * Workhorse function for dsa_attach and dsa_attach_in_place.
 */
static dsa_area *
attach_internal(void *place, dsm_segment *segment, dsa_handle handle)
{
    dsa_area_control *control;
    dsa_area   *area;
    dsa_segment_map *segment_map;

    control = (dsa_area_control *) place;
    Assert(control->handle == handle);
    Assert(control->segment_handles[0] == handle);
    Assert(control->segment_header.magic ==
           (DSA_SEGMENT_HEADER_MAGIC ^ handle ^ 0));

    /* Build the backend-local area object. */
    area = palloc(sizeof(dsa_area));
    area->control = control;
    area->mapping_pinned = false;
    memset(&area->segment_maps[0], 0,
           sizeof(dsa_segment_map) * DSA_MAX_SEGMENTS);
    area->high_segment_index = 0;

    /* Set up the segment map for this process's mapping. */
    segment_map = &area->segment_maps[0];
    segment_map->segment = segment; /* NULL for in-place */
    segment_map->mapped_address = place;
    segment_map->header = (dsa_segment_header *) segment_map->mapped_address;
    segment_map->fpm = (FreePageManager *)
        (segment_map->mapped_address + MAXALIGN(sizeof(dsa_area_control)));
    segment_map->pagemap = (dsa_pointer *)
        (segment_map->mapped_address + MAXALIGN(sizeof(dsa_area_control)) +
         MAXALIGN(sizeof(FreePageManager)));

    /* Bump the reference count. */
    LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
    if (control->refcnt == 0)
    {
        /* We can't attach to a DSA area that has already been destroyed. */
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("could not attach to dynamic shared area")));
    }
    ++control->refcnt;
    area->freed_segment_counter = area->control->freed_segment_counter;
    LWLockRelease(DSA_AREA_LOCK(area));

    return area;
}

/*
 * Add a new span to fullness class 1 of the indicated pool.
 */
static void
init_span(dsa_area *area,
          dsa_pointer span_pointer,
          dsa_area_pool *pool, dsa_pointer start, size_t npages,
          uint16 size_class)
{
    dsa_area_span *span = dsa_get_address(area, span_pointer);
    size_t      obsize = dsa_size_classes[size_class];

    /*
     * The per-pool lock must be held because we manipulate the span list for
     * this pool.
     */
    Assert(LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));

    /* Push this span onto the front of the span list for fullness class 1. */
    if (DsaPointerIsValid(pool->spans[1]))
    {
        dsa_area_span *head = (dsa_area_span *)
        dsa_get_address(area, pool->spans[1]);

        head->prevspan = span_pointer;
    }
    span->pool = DsaAreaPoolToDsaPointer(area, pool);
    span->nextspan = pool->spans[1];
    span->prevspan = InvalidDsaPointer;
    pool->spans[1] = span_pointer;

    span->start = start;
    span->npages = npages;
    span->size_class = size_class;
    span->ninitialized = 0;
    if (size_class == DSA_SCLASS_BLOCK_OF_SPANS)
    {
        /*
         * A block-of-spans contains its own descriptor, so mark one object as
         * initialized and reduce the count of allocatable objects by one.
         * Doing this here has the side effect of also reducing nmax by one,
         * which is important to make sure we free this object at the correct
         * time.
         */
        span->ninitialized = 1;
        span->nallocatable = FPM_PAGE_SIZE / obsize - 1;
    }
    else if (size_class != DSA_SCLASS_SPAN_LARGE)
        span->nallocatable = DSA_SUPERBLOCK_SIZE / obsize;
    span->firstfree = DSA_SPAN_NOTHING_FREE;
    span->nmax = span->nallocatable;
    span->fclass = 1;
}

/*
 * Transfer the first span in one fullness class to the head of another
 * fullness class.
 */
static bool
transfer_first_span(dsa_area *area,
                    dsa_area_pool *pool, int fromclass, int toclass)
{
    dsa_pointer span_pointer;
    dsa_area_span *span;
    dsa_area_span *nextspan;

    /* Can't do it if source list is empty. */
    span_pointer = pool->spans[fromclass];
    if (!DsaPointerIsValid(span_pointer))
        return false;

    /* Remove span from head of source list. */
    span = dsa_get_address(area, span_pointer);
    pool->spans[fromclass] = span->nextspan;
    if (DsaPointerIsValid(span->nextspan))
    {
        nextspan = (dsa_area_span *)
            dsa_get_address(area, span->nextspan);
        nextspan->prevspan = InvalidDsaPointer;
    }

    /* Add span to head of target list. */
    span->nextspan = pool->spans[toclass];
    pool->spans[toclass] = span_pointer;
    if (DsaPointerIsValid(span->nextspan))
    {
        nextspan = (dsa_area_span *)
            dsa_get_address(area, span->nextspan);
        nextspan->prevspan = span_pointer;
    }
    span->fclass = toclass;

    return true;
}

/*
 * Allocate one object of the requested size class from the given area.
 */
static inline dsa_pointer
alloc_object(dsa_area *area, int size_class)
{
    dsa_area_pool *pool = &area->control->pools[size_class];
    dsa_area_span *span;
    dsa_pointer block;
    dsa_pointer result;
    char       *object;
    size_t      size;

    /*
     * Even though ensure_active_superblock can in turn call alloc_object if
     * it needs to allocate a new span, that's always from a different pool,
     * and the order of lock acquisition is always the same, so it's OK that
     * we hold this lock for the duration of this function.
     */
    Assert(!LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));
    LWLockAcquire(DSA_SCLASS_LOCK(area, size_class), LW_EXCLUSIVE);

    /*
     * If there's no active superblock, we must successfully obtain one or
     * fail the request.
     */
    if (!DsaPointerIsValid(pool->spans[1]) &&
        !ensure_active_superblock(area, pool, size_class))
    {
        result = InvalidDsaPointer;
    }
    else
    {
        /*
         * There should be a block in fullness class 1 at this point, and it
         * should never be completely full.  Thus we can either pop an object
         * from the free list or, failing that, initialize a new object.
         */
        Assert(DsaPointerIsValid(pool->spans[1]));
        span = (dsa_area_span *)
            dsa_get_address(area, pool->spans[1]);
        Assert(span->nallocatable > 0);
        block = span->start;
        Assert(size_class < DSA_NUM_SIZE_CLASSES);
        size = dsa_size_classes[size_class];
        if (span->firstfree != DSA_SPAN_NOTHING_FREE)
        {
            result = block + span->firstfree * size;
            object = dsa_get_address(area, result);
            span->firstfree = NextFreeObjectIndex(object);
        }
        else
        {
            result = block + span->ninitialized * size;
            ++span->ninitialized;
        }
        --span->nallocatable;

        /* If it's now full, move it to the highest-numbered fullness class. */
        if (span->nallocatable == 0)
            transfer_first_span(area, pool, 1, DSA_FULLNESS_CLASSES - 1);
    }

    Assert(LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));
    LWLockRelease(DSA_SCLASS_LOCK(area, size_class));

    return result;
}

/*
 * Ensure an active (i.e. fullness class 1) superblock, unless all existing
 * superblocks are completely full and no more can be allocated.
 *
 * Fullness classes K of 0..N are loosely intended to represent blocks whose
 * utilization percentage is at least K/N, but we only enforce this rigorously
 * for the highest-numbered fullness class, which always contains exactly
 * those blocks that are completely full.  It's otherwise acceptable for a
 * block to be in a higher-numbered fullness class than the one to which it
 * logically belongs.  In addition, the active block, which is always the
 * first block in fullness class 1, is permitted to have a higher allocation
 * percentage than would normally be allowable for that fullness class; we
 * don't move it until it's completely full, and then it goes to the
 * highest-numbered fullness class.
 *
 * It might seem odd that the active block is the head of fullness class 1
 * rather than fullness class 0, but experience with other allocators has
 * shown that it's usually better to allocate from a block that's moderately
 * full rather than one that's nearly empty.  Insofar as is reasonably
 * possible, we want to avoid performing new allocations in a block that would
 * otherwise become empty soon.
 */
static bool
ensure_active_superblock(dsa_area *area, dsa_area_pool *pool,
                         int size_class)
{
    dsa_pointer span_pointer;
    dsa_pointer start_pointer;
    size_t      obsize = dsa_size_classes[size_class];
    size_t      nmax;
    int         fclass;
    size_t      npages = 1;
    size_t      first_page;
    size_t      i;
    dsa_segment_map *segment_map;

    Assert(LWLockHeldByMe(DSA_SCLASS_LOCK(area, size_class)));

    /*
     * Compute the number of objects that will fit in a block of this size
     * class.  Span-of-spans blocks are just a single page, and the first
     * object isn't available for use because it describes the block-of-spans
     * itself.
     */
    if (size_class == DSA_SCLASS_BLOCK_OF_SPANS)
        nmax = FPM_PAGE_SIZE / obsize - 1;
    else
        nmax = DSA_SUPERBLOCK_SIZE / obsize;

    /*
     * If fullness class 1 is empty, try to find a span to put in it by
     * scanning higher-numbered fullness classes (excluding the last one,
     * whose blocks are certain to all be completely full).
     */
    for (fclass = 2; fclass < DSA_FULLNESS_CLASSES - 1; ++fclass)
    {
        span_pointer = pool->spans[fclass];

        while (DsaPointerIsValid(span_pointer))
        {
            int         tfclass;
            dsa_area_span *span;
            dsa_area_span *nextspan;
            dsa_area_span *prevspan;
            dsa_pointer next_span_pointer;

            span = (dsa_area_span *)
                dsa_get_address(area, span_pointer);
            next_span_pointer = span->nextspan;

            /* Figure out what fullness class should contain this span. */
            tfclass = (nmax - span->nallocatable)
                * (DSA_FULLNESS_CLASSES - 1) / nmax;

            /* Look up next span. */
            if (DsaPointerIsValid(span->nextspan))
                nextspan = (dsa_area_span *)
                    dsa_get_address(area, span->nextspan);
            else
                nextspan = NULL;

            /*
             * If utilization has dropped enough that this now belongs in some
             * other fullness class, move it there.
             */
            if (tfclass < fclass)
            {
                /* Remove from the current fullness class list. */
                if (pool->spans[fclass] == span_pointer)
                {
                    /* It was the head; remove it. */
                    Assert(!DsaPointerIsValid(span->prevspan));
                    pool->spans[fclass] = span->nextspan;
                    if (nextspan != NULL)
                        nextspan->prevspan = InvalidDsaPointer;
                }
                else
                {
                    /* It was not the head. */
                    Assert(DsaPointerIsValid(span->prevspan));
                    prevspan = (dsa_area_span *)
                        dsa_get_address(area, span->prevspan);
                    prevspan->nextspan = span->nextspan;
                }
                if (nextspan != NULL)
                    nextspan->prevspan = span->prevspan;

                /* Push onto the head of the new fullness class list. */
                span->nextspan = pool->spans[tfclass];
                pool->spans[tfclass] = span_pointer;
                span->prevspan = InvalidDsaPointer;
                if (DsaPointerIsValid(span->nextspan))
                {
                    nextspan = (dsa_area_span *)
                        dsa_get_address(area, span->nextspan);
                    nextspan->prevspan = span_pointer;
                }
                span->fclass = tfclass;
            }

            /* Advance to next span on list. */
            span_pointer = next_span_pointer;
        }

        /* Stop now if we found a suitable block. */
        if (DsaPointerIsValid(pool->spans[1]))
            return true;
    }

    /*
     * If there are no blocks that properly belong in fullness class 1, pick
     * one from some other fullness class and move it there anyway, so that we
     * have an allocation target.  Our last choice is to transfer a block
     * that's almost empty (and might become completely empty soon if left
     * alone), but even that is better than failing, which is what we must do
     * if there are no blocks at all with freespace.
     */
    Assert(!DsaPointerIsValid(pool->spans[1]));
    for (fclass = 2; fclass < DSA_FULLNESS_CLASSES - 1; ++fclass)
        if (transfer_first_span(area, pool, fclass, 1))
            return true;
    if (!DsaPointerIsValid(pool->spans[1]) &&
        transfer_first_span(area, pool, 0, 1))
        return true;

    /*
     * We failed to find an existing span with free objects, so we need to
     * allocate a new superblock and construct a new span to manage it.
     *
     * First, get a dsa_area_span object to describe the new superblock block
     * ... unless this allocation is for a dsa_area_span object, in which case
     * that's surely not going to work.  We handle that case by storing the
     * span describing a block-of-spans inline.
     */
    if (size_class != DSA_SCLASS_BLOCK_OF_SPANS)
    {
        span_pointer = alloc_object(area, DSA_SCLASS_BLOCK_OF_SPANS);
        if (!DsaPointerIsValid(span_pointer))
            return false;
        npages = DSA_PAGES_PER_SUPERBLOCK;
    }

    /* Find or create a segment and allocate the superblock. */
    LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
    segment_map = get_best_segment(area, npages);
    if (segment_map == NULL)
    {
        segment_map = make_new_segment(area, npages);
        if (segment_map == NULL)
        {
            LWLockRelease(DSA_AREA_LOCK(area));
            return false;
        }
    }

    /*
     * This shouldn't happen: get_best_segment() or make_new_segment()
     * promised that we can successfully allocate npages.
     */
    if (!FreePageManagerGet(segment_map->fpm, npages, &first_page))
        elog(FATAL,
             "dsa_allocate could not find %zu free pages for superblock",
             npages);
    LWLockRelease(DSA_AREA_LOCK(area));

    /* Compute the start of the superblock. */
    start_pointer =
        DSA_MAKE_POINTER(get_segment_index(area, segment_map),
                         first_page * FPM_PAGE_SIZE);

    /*
     * If this is a block-of-spans, carve the descriptor right out of the
     * allocated space.
     */
    if (size_class == DSA_SCLASS_BLOCK_OF_SPANS)
    {
        /*
         * We have a pointer into the segment.  We need to build a dsa_pointer
         * from the segment index and offset into the segment.
         */
        span_pointer = start_pointer;
    }

    /* Initialize span and pagemap. */
    init_span(area, span_pointer, pool, start_pointer, npages, size_class);
    for (i = 0; i < npages; ++i)
        segment_map->pagemap[first_page + i] = span_pointer;

    return true;
}

/*
 * Return the segment map corresponding to a given segment index, mapping the
 * segment in if necessary.  For internal segment book-keeping, this is called
 * with the area lock held.  It is also called by dsa_free and dsa_get_address
 * without any locking, relying on the fact they have a known live segment
 * index and they always call check_for_freed_segments to ensures that any
 * freed segment occupying the same slot is detached first.
 */
static dsa_segment_map *
get_segment_by_index(dsa_area *area, dsa_segment_index index)
{
    if (unlikely(area->segment_maps[index].mapped_address == NULL))
    {
        dsm_handle  handle;
        dsm_segment *segment;
        dsa_segment_map *segment_map;

        /*
         * If we are reached by dsa_free or dsa_get_address, there must be at
         * least one object allocated in the referenced segment.  Otherwise,
         * their caller has a double-free or access-after-free bug, which we
         * have no hope of detecting.  So we know it's safe to access this
         * array slot without holding a lock; it won't change underneath us.
         * Furthermore, we know that we can see the latest contents of the
         * slot, as explained in check_for_freed_segments, which those
         * functions call before arriving here.
         */
        handle = area->control->segment_handles[index];

        /* It's an error to try to access an unused slot. */
        if (handle == DSM_HANDLE_INVALID)
            elog(ERROR,
                 "dsa_area could not attach to a segment that has been freed");

        segment = dsm_attach(handle);
        if (segment == NULL)
            elog(ERROR, "dsa_area could not attach to segment");
        if (area->mapping_pinned)
            dsm_pin_mapping(segment);
        segment_map = &area->segment_maps[index];
        segment_map->segment = segment;
        segment_map->mapped_address = dsm_segment_address(segment);
        segment_map->header =
            (dsa_segment_header *) segment_map->mapped_address;
        segment_map->fpm = (FreePageManager *)
            (segment_map->mapped_address +
             MAXALIGN(sizeof(dsa_segment_header)));
        segment_map->pagemap = (dsa_pointer *)
            (segment_map->mapped_address +
             MAXALIGN(sizeof(dsa_segment_header)) +
             MAXALIGN(sizeof(FreePageManager)));

        /* Remember the highest index this backend has ever mapped. */
        if (area->high_segment_index < index)
            area->high_segment_index = index;

        Assert(segment_map->header->magic ==
               (DSA_SEGMENT_HEADER_MAGIC ^ area->control->handle ^ index));
    }

    /*
     * Callers of dsa_get_address() and dsa_free() don't hold the area lock,
     * but it's a bug in the calling code and undefined behavior if the
     * address is not live (ie if the segment might possibly have been freed,
     * they're trying to use a dangling pointer).
     *
     * For dsa.c code that holds the area lock to manipulate segment_bins
     * lists, it would be a bug if we ever reach a freed segment here.  After
     * it's marked as freed, the only thing any backend should do with it is
     * unmap it, and it should always have done that in
     * check_for_freed_segments_locked() before arriving here to resolve an
     * index to a segment_map.
     *
     * Either way we can assert that we aren't returning a freed segment.
     */
    Assert(!area->segment_maps[index].header->freed);

    return &area->segment_maps[index];
}

/*
 * Return a superblock to the free page manager.  If the underlying segment
 * has become entirely free, then return it to the operating system.
 *
 * The appropriate pool lock must be held.
 */
static void
destroy_superblock(dsa_area *area, dsa_pointer span_pointer)
{
    dsa_area_span *span = dsa_get_address(area, span_pointer);
    int         size_class = span->size_class;
    dsa_segment_map *segment_map;


    /* Remove it from its fullness class list. */
    unlink_span(area, span);

    /*
     * Note: Here we acquire the area lock while we already hold a per-pool
     * lock.  We never hold the area lock and then take a pool lock, or we
     * could deadlock.
     */
    LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
    check_for_freed_segments_locked(area);
    segment_map =
        get_segment_by_index(area, DSA_EXTRACT_SEGMENT_NUMBER(span->start));
    FreePageManagerPut(segment_map->fpm,
                       DSA_EXTRACT_OFFSET(span->start) / FPM_PAGE_SIZE,
                       span->npages);
    /* Check if the segment is now entirely free. */
    if (fpm_largest(segment_map->fpm) == segment_map->header->usable_pages)
    {
        dsa_segment_index index = get_segment_index(area, segment_map);

        /* If it's not the segment with extra control data, free it. */
        if (index != 0)
        {
            /*
             * Give it back to the OS, and allow other backends to detect that
             * they need to detach.
             */
            unlink_segment(area, segment_map);
            segment_map->header->freed = true;
            Assert(area->control->total_segment_size >=
                   segment_map->header->size);
            area->control->total_segment_size -=
                segment_map->header->size;
            dsm_unpin_segment(dsm_segment_handle(segment_map->segment));
            dsm_detach(segment_map->segment);
            area->control->segment_handles[index] = DSM_HANDLE_INVALID;
            ++area->control->freed_segment_counter;
            segment_map->segment = NULL;
            segment_map->header = NULL;
            segment_map->mapped_address = NULL;
        }
    }
    LWLockRelease(DSA_AREA_LOCK(area));

    /*
     * Span-of-spans blocks store the span which describes them within the
     * block itself, so freeing the storage implicitly frees the descriptor
     * also.  If this is a block of any other type, we need to separately free
     * the span object also.  This recursive call to dsa_free will acquire the
     * span pool's lock.  We can't deadlock because the acquisition order is
     * always some other pool and then the span pool.
     */
    if (size_class != DSA_SCLASS_BLOCK_OF_SPANS)
        dsa_free(area, span_pointer);
}

static void
unlink_span(dsa_area *area, dsa_area_span *span)
{
    if (DsaPointerIsValid(span->nextspan))
    {
        dsa_area_span *next = dsa_get_address(area, span->nextspan);

        next->prevspan = span->prevspan;
    }
    if (DsaPointerIsValid(span->prevspan))
    {
        dsa_area_span *prev = dsa_get_address(area, span->prevspan);

        prev->nextspan = span->nextspan;
    }
    else
    {
        dsa_area_pool *pool = dsa_get_address(area, span->pool);

        pool->spans[span->fclass] = span->nextspan;
    }
}

static void
add_span_to_fullness_class(dsa_area *area, dsa_area_span *span,
                           dsa_pointer span_pointer,
                           int fclass)
{
    dsa_area_pool *pool = dsa_get_address(area, span->pool);

    if (DsaPointerIsValid(pool->spans[fclass]))
    {
        dsa_area_span *head = dsa_get_address(area,
                                              pool->spans[fclass]);

        head->prevspan = span_pointer;
    }
    span->prevspan = InvalidDsaPointer;
    span->nextspan = pool->spans[fclass];
    pool->spans[fclass] = span_pointer;
    span->fclass = fclass;
}

/*
 * Detach from an area that was either created or attached to by this process.
 */
void
dsa_detach(dsa_area *area)
{
    int         i;

    /* Detach from all segments. */
    for (i = 0; i <= area->high_segment_index; ++i)
        if (area->segment_maps[i].segment != NULL)
            dsm_detach(area->segment_maps[i].segment);

    /*
     * Note that 'detaching' (= detaching from DSM segments) doesn't include
     * 'releasing' (= adjusting the reference count).  It would be nice to
     * combine these operations, but client code might never get around to
     * calling dsa_detach because of an error path, and a detach hook on any
     * particular segment is too late to detach other segments in the area
     * without risking a 'leak' warning in the non-error path.
     */

    /* Free the backend-local area object. */
    pfree(area);
}

/*
 * Unlink a segment from the bin that contains it.
 */
static void
unlink_segment(dsa_area *area, dsa_segment_map *segment_map)
{
    if (segment_map->header->prev != DSA_SEGMENT_INDEX_NONE)
    {
        dsa_segment_map *prev;

        prev = get_segment_by_index(area, segment_map->header->prev);
        prev->header->next = segment_map->header->next;
    }
    else
    {
        Assert(area->control->segment_bins[segment_map->header->bin] ==
               get_segment_index(area, segment_map));
        area->control->segment_bins[segment_map->header->bin] =
            segment_map->header->next;
    }
    if (segment_map->header->next != DSA_SEGMENT_INDEX_NONE)
    {
        dsa_segment_map *next;

        next = get_segment_by_index(area, segment_map->header->next);
        next->header->prev = segment_map->header->prev;
    }
}

/*
 * Find a segment that could satisfy a request for 'npages' of contiguous
 * memory, or return NULL if none can be found.  This may involve attaching to
 * segments that weren't previously attached so that we can query their free
 * pages map.
 */
static dsa_segment_map *
get_best_segment(dsa_area *area, size_t npages)
{
    size_t      bin;

    Assert(LWLockHeldByMe(DSA_AREA_LOCK(area)));
    check_for_freed_segments_locked(area);

    /*
     * Start searching from the first bin that *might* have enough contiguous
     * pages.
     */
    for (bin = contiguous_pages_to_segment_bin(npages);
         bin < DSA_NUM_SEGMENT_BINS;
         ++bin)
    {
        /*
         * The minimum contiguous size that any segment in this bin should
         * have.  We'll re-bin if we see segments with fewer.
         */
        size_t      threshold = (size_t) 1 << (bin - 1);
        dsa_segment_index segment_index;

        /* Search this bin for a segment with enough contiguous space. */
        segment_index = area->control->segment_bins[bin];
        while (segment_index != DSA_SEGMENT_INDEX_NONE)
        {
            dsa_segment_map *segment_map;
            dsa_segment_index next_segment_index;
            size_t      contiguous_pages;

            segment_map = get_segment_by_index(area, segment_index);
            next_segment_index = segment_map->header->next;
            contiguous_pages = fpm_largest(segment_map->fpm);

            /* Not enough for the request, still enough for this bin. */
            if (contiguous_pages >= threshold && contiguous_pages < npages)
            {
                segment_index = next_segment_index;
                continue;
            }

            /* Re-bin it if it's no longer in the appropriate bin. */
            if (contiguous_pages < threshold)
            {
                size_t      new_bin;

                new_bin = contiguous_pages_to_segment_bin(contiguous_pages);

                /* Remove it from its current bin. */
                unlink_segment(area, segment_map);

                /* Push it onto the front of its new bin. */
                segment_map->header->prev = DSA_SEGMENT_INDEX_NONE;
                segment_map->header->next =
                    area->control->segment_bins[new_bin];
                segment_map->header->bin = new_bin;
                area->control->segment_bins[new_bin] = segment_index;
                if (segment_map->header->next != DSA_SEGMENT_INDEX_NONE)
                {
                    dsa_segment_map *next;

                    next = get_segment_by_index(area,
                                                segment_map->header->next);
                    Assert(next->header->bin == new_bin);
                    next->header->prev = segment_index;
                }

                /*
                 * But fall through to see if it's enough to satisfy this
                 * request anyway....
                 */
            }

            /* Check if we are done. */
            if (contiguous_pages >= npages)
                return segment_map;

            /* Continue searching the same bin. */
            segment_index = next_segment_index;
        }
    }

    /* Not found. */
    return NULL;
}

/*
 * Create a new segment that can handle at least requested_pages.  Returns
 * NULL if the requested total size limit or maximum allowed number of
 * segments would be exceeded.
 */
static dsa_segment_map *
make_new_segment(dsa_area *area, size_t requested_pages)
{
    dsa_segment_index new_index;
    size_t      metadata_bytes;
    size_t      total_size;
    size_t      total_pages;
    size_t      usable_pages;
    dsa_segment_map *segment_map;
    dsm_segment *segment;

    Assert(LWLockHeldByMe(DSA_AREA_LOCK(area)));

    /* Find a segment slot that is not in use (linearly for now). */
    for (new_index = 1; new_index < DSA_MAX_SEGMENTS; ++new_index)
    {
        if (area->control->segment_handles[new_index] == DSM_HANDLE_INVALID)
            break;
    }
    if (new_index == DSA_MAX_SEGMENTS)
        return NULL;

    /*
     * If the total size limit is already exceeded, then we exit early and
     * avoid arithmetic wraparound in the unsigned expressions below.
     */
    if (area->control->total_segment_size >=
        area->control->max_total_segment_size)
        return NULL;

    /*
     * The size should be at least as big as requested, and at least big
     * enough to follow a geometric series that approximately doubles the
     * total storage each time we create a new segment.  We use geometric
     * growth because the underlying DSM system isn't designed for large
     * numbers of segments (otherwise we might even consider just using one
     * DSM segment for each large allocation and for each superblock, and then
     * we wouldn't need to use FreePageManager).
     *
     * We decide on a total segment size first, so that we produce tidy
     * power-of-two sized segments.  This is a good property to have if we
     * move to huge pages in the future.  Then we work back to the number of
     * pages we can fit.
     */
    total_size = DSA_INITIAL_SEGMENT_SIZE *
        ((size_t) 1 << (new_index / DSA_NUM_SEGMENTS_AT_EACH_SIZE));
    total_size = Min(total_size, DSA_MAX_SEGMENT_SIZE);
    total_size = Min(total_size,
                     area->control->max_total_segment_size -
                     area->control->total_segment_size);

    total_pages = total_size / FPM_PAGE_SIZE;
    metadata_bytes =
        MAXALIGN(sizeof(dsa_segment_header)) +
        MAXALIGN(sizeof(FreePageManager)) +
        sizeof(dsa_pointer) * total_pages;

    /* Add padding up to next page boundary. */
    if (metadata_bytes % FPM_PAGE_SIZE != 0)
        metadata_bytes += FPM_PAGE_SIZE - (metadata_bytes % FPM_PAGE_SIZE);
    if (total_size <= metadata_bytes)
        return NULL;
    usable_pages = (total_size - metadata_bytes) / FPM_PAGE_SIZE;
    Assert(metadata_bytes + usable_pages * FPM_PAGE_SIZE <= total_size);

    /* See if that is enough... */
    if (requested_pages > usable_pages)
    {
        /*
         * We'll make an odd-sized segment, working forward from the requested
         * number of pages.
         */
        usable_pages = requested_pages;
        metadata_bytes =
            MAXALIGN(sizeof(dsa_segment_header)) +
            MAXALIGN(sizeof(FreePageManager)) +
            usable_pages * sizeof(dsa_pointer);

        /* Add padding up to next page boundary. */
        if (metadata_bytes % FPM_PAGE_SIZE != 0)
            metadata_bytes += FPM_PAGE_SIZE - (metadata_bytes % FPM_PAGE_SIZE);
        total_size = metadata_bytes + usable_pages * FPM_PAGE_SIZE;

        /* Is that too large for dsa_pointer's addressing scheme? */
        if (total_size > DSA_MAX_SEGMENT_SIZE)
            return NULL;

        /* Would that exceed the limit? */
        if (total_size > area->control->max_total_segment_size -
            area->control->total_segment_size)
            return NULL;
    }

    /* Create the segment. */
    segment = dsm_create(total_size, 0);
    if (segment == NULL)
        return NULL;
    dsm_pin_segment(segment);
    if (area->mapping_pinned)
        dsm_pin_mapping(segment);

    /* Store the handle in shared memory to be found by index. */
    area->control->segment_handles[new_index] =
        dsm_segment_handle(segment);
    /* Track the highest segment index in the history of the area. */
    if (area->control->high_segment_index < new_index)
        area->control->high_segment_index = new_index;
    /* Track the highest segment index this backend has ever mapped. */
    if (area->high_segment_index < new_index)
        area->high_segment_index = new_index;
    /* Track total size of all segments. */
    area->control->total_segment_size += total_size;
    Assert(area->control->total_segment_size <=
           area->control->max_total_segment_size);

    /* Build a segment map for this segment in this backend. */
    segment_map = &area->segment_maps[new_index];
    segment_map->segment = segment;
    segment_map->mapped_address = dsm_segment_address(segment);
    segment_map->header = (dsa_segment_header *) segment_map->mapped_address;
    segment_map->fpm = (FreePageManager *)
        (segment_map->mapped_address +
         MAXALIGN(sizeof(dsa_segment_header)));
    segment_map->pagemap = (dsa_pointer *)
        (segment_map->mapped_address +
         MAXALIGN(sizeof(dsa_segment_header)) +
         MAXALIGN(sizeof(FreePageManager)));

    /* Set up the free page map. */
    FreePageManagerInitialize(segment_map->fpm, segment_map->mapped_address);
    FreePageManagerPut(segment_map->fpm, metadata_bytes / FPM_PAGE_SIZE,
                       usable_pages);

    /* Set up the segment header and put it in the appropriate bin. */
    segment_map->header->magic =
        DSA_SEGMENT_HEADER_MAGIC ^ area->control->handle ^ new_index;
    segment_map->header->usable_pages = usable_pages;
    segment_map->header->size = total_size;
    segment_map->header->bin = contiguous_pages_to_segment_bin(usable_pages);
    segment_map->header->prev = DSA_SEGMENT_INDEX_NONE;
    segment_map->header->next =
        area->control->segment_bins[segment_map->header->bin];
    segment_map->header->freed = false;
    area->control->segment_bins[segment_map->header->bin] = new_index;
    if (segment_map->header->next != DSA_SEGMENT_INDEX_NONE)
    {
        dsa_segment_map *next =
        get_segment_by_index(area, segment_map->header->next);

        Assert(next->header->bin == segment_map->header->bin);
        next->header->prev = new_index;
    }

    return segment_map;
}

/*
 * Check if any segments have been freed by destroy_superblock, so we can
 * detach from them in this backend.  This function is called by
 * dsa_get_address and dsa_free to make sure that a dsa_pointer they have
 * received can be resolved to the correct segment.
 *
 * The danger we want to defend against is that there could be an old segment
 * mapped into a given slot in this backend, and the dsa_pointer they have
 * might refer to some new segment in the same slot.  So those functions must
 * be sure to process all instructions to detach from a freed segment that had
 * been generated by the time this process received the dsa_pointer, before
 * they call get_segment_by_index.
 */
static void
check_for_freed_segments(dsa_area *area)
{
    size_t      freed_segment_counter;

    /*
     * Any other process that has freed a segment has incremented
     * freed_segment_counter while holding an LWLock, and that must precede
     * any backend creating a new segment in the same slot while holding an
     * LWLock, and that must precede the creation of any dsa_pointer pointing
     * into the new segment which might reach us here, and the caller must
     * have sent the dsa_pointer to this process using appropriate memory
     * synchronization (some kind of locking or atomic primitive or system
     * call).  So all we need to do on the reading side is ask for the load of
     * freed_segment_counter to follow the caller's load of the dsa_pointer it
     * has, and we can be sure to detect any segments that had been freed as
     * of the time that the dsa_pointer reached this process.
     */
    pg_read_barrier();
    freed_segment_counter = area->control->freed_segment_counter;
    if (unlikely(area->freed_segment_counter != freed_segment_counter))
    {
        /* Check all currently mapped segments to find what's been freed. */
        LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
        check_for_freed_segments_locked(area);
        LWLockRelease(DSA_AREA_LOCK(area));
    }
}

/*
 * Workhorse for check_for_freed_segments(), and also used directly in path
 * where the area lock is already held.  This should be called after acquiring
 * the lock but before looking up any segment by index number, to make sure we
 * unmap any stale segments that might have previously had the same index as a
 * current segment.
 */
static void
check_for_freed_segments_locked(dsa_area *area)
{
    size_t      freed_segment_counter;
    int         i;

    Assert(LWLockHeldByMe(DSA_AREA_LOCK(area)));
    freed_segment_counter = area->control->freed_segment_counter;
    if (unlikely(area->freed_segment_counter != freed_segment_counter))
    {
        for (i = 0; i <= area->high_segment_index; ++i)
        {
            if (area->segment_maps[i].header != NULL &&
                area->segment_maps[i].header->freed)
            {
                dsm_detach(area->segment_maps[i].segment);
                area->segment_maps[i].segment = NULL;
                area->segment_maps[i].header = NULL;
                area->segment_maps[i].mapped_address = NULL;
            }
        }
        area->freed_segment_counter = freed_segment_counter;
    }
}