shmem.c

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/*-------------------------------------------------------------------------
 *
 * shmem.c
 *    create shared memory and initialize shared memory data structures.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/shmem.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * POSTGRES processes share one or more regions of shared memory.
 * The shared memory is created by a postmaster and is inherited
 * by each backend via fork() (or, in some ports, via other OS-specific
 * methods).  The routines in this file are used for allocating and
 * binding to shared memory data structures.
 *
 * This module provides facilities to allocate fixed-size structures in shared
 * memory, for things like variables shared between all backend processes.
 * Each such structure has a string name to identify it, specified when it is
 * requested.  shmem_hash.c provides a shared hash table implementation on top
 * of that.
 *
 * Shared memory areas should usually not be allocated after postmaster
 * startup, although we do allow small allocations later for the benefit of
 * extension modules that are loaded after startup.  Despite that allowance,
 * extensions that need shared memory should be added in
 * shared_preload_libraries, because the allowance is quite small and there is
 * no guarantee that any memory is available after startup.
 *
 * Nowadays, there is also another way to allocate shared memory called
 * Dynamic Shared Memory.  See dsm.c for that facility.  One big difference
 * between traditional shared memory handled by shmem.c and dynamic shared
 * memory is that traditional shared memory areas are mapped to the same
 * address in all processes, so you can use normal pointers in shared memory
 * structs.  With Dynamic Shared Memory, you must use offsets or DSA pointers
 * instead.
 *
 * Shared memory managed by shmem.c can never be freed, once allocated.  Each
 * hash table has its own free list, so hash buckets can be reused when an
 * item is deleted.
 *
 * Usage
 * -----
 *
 * To allocate shared memory, you need to register a set of callback functions
 * which handle the lifecycle of the allocation.  In the request_fn callback,
 * call ShmemRequestStruct() with the desired name and size.  When the area is
 * later allocated or attached to, the global variable pointed to by the .ptr
 * option is set to the shared memory location of the allocation.  The init_fn
 * callback can perform additional initialization.
 *
 *  typedef struct MyShmemData {
 *      ...
 *  } MyShmemData;
 *
 *  static MyShmemData *MyShmem;
 *
 *  static void my_shmem_request(void *arg);
 *  static void my_shmem_init(void *arg);
 *
 *  const ShmemCallbacks MyShmemCallbacks = {
 *      .request_fn = my_shmem_request,
 *      .init_fn = my_shmem_init,
 *  };
 *
 *  static void
 *  my_shmem_request(void *arg)
 *  {
 *      ShmemRequestStruct(.name = "My shmem area",
 *                         .size = sizeof(MyShmemData),
 *                         .ptr = (void **) &MyShmem,
 *          );
 *  }
 *
 * In builtin PostgreSQL code, add the callbacks to the list in
 * src/include/storage/subsystemlist.h.  In an add-in module, you can register
 * the callbacks by calling RegisterShmemCallbacks(&MyShmemCallbacks) in the
 * extension's _PG_init() function.
 *
 * Lifecycle
 * ---------
 *
 * Initializing shared memory happens in multiple phases.  In the first phase,
 * during postmaster startup, all the request_fn callbacks are called.  Only
 * after all the request_fn callbacks have been called and all the shmem areas
 * have been requested by the ShmemRequestStruct() calls we know how much
 * shared memory we need in total.  After that, postmaster allocates global
 * shared memory segment, and calls all the init_fn callbacks to initialize
 * all the requested shmem areas.
 *
 * In standard Unix-ish environments, individual backends do not need to
 * re-establish their local pointers into shared memory, because they inherit
 * correct values of those variables via fork() from the postmaster.  However,
 * this does not work in the EXEC_BACKEND case.  In ports using EXEC_BACKEND,
 * backend startup also calls the shmem_request callbacks to re-establish the
 * knowledge about each shared memory area, sets the pointer variables
 * (*options->ptr), and calls the attach_fn callback, if any, for additional
 * per-backend setup.
 *
 * Legacy ShmemInitStruct()/ShmemInitHash() functions
 * --------------------------------------------------
 *
 * ShmemInitStruct()/ShmemInitHash() is another way of registering shmem
 * areas.  It pre-dates the ShmemRequestStruct()/ShmemRequestHash() functions,
 * and should not be used in new code, but as of this writing it is still
 * widely used in extensions.
 *
 * To allocate a shmem area with ShmemInitStruct(), you need to separately
 * register the size needed for the area by calling RequestAddinShmemSpace()
 * from the extension's shmem_request_hook, and allocate the area by calling
 * ShmemInitStruct() from the extension's shmem_startup_hook.  There are no
 * init/attach callbacks.  Instead, the caller of ShmemInitStruct() must check
 * the return status of ShmemInitStruct() and initialize the struct if it was
 * not previously initialized.
 *
 * Calling ShmemAlloc() directly
 * -----------------------------
 *
 * There's a more low-level way of allocating shared memory too: you can call
 * ShmemAlloc() directly.  It's used to implement the higher level mechanisms,
 * and should generally not be called directly.
 */
 
#include "postgres.h"
 
#include <unistd.h>
 
#include "access/slru.h"
#include "fmgr.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "port/pg_bitutils.h"
#include "port/pg_numa.h"
#include "storage/lwlock.h"
#include "storage/pg_shmem.h"
#include "storage/shmem.h"
#include "storage/shmem_internal.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/tuplestore.h"
 
/*
 * Registered callbacks.
 *
 * During postmaster startup, we accumulate the callbacks from all subsystems
 * in this list.
 *
 * This is in process private memory, although on Unix-like systems, we expect
 * all the registrations to happen at postmaster startup time and be inherited
 * by all the child processes via fork().
 */
static List *registered_shmem_callbacks;
 
/*
 * In the shmem request phase, all the shmem areas requested with the
 * ShmemRequest*() functions are accumulated here.
 */
typedef struct
{
    ShmemStructOpts *options;
    ShmemRequestKind kind;
} ShmemRequest;
 
static List *pending_shmem_requests;
 
/*
 * Per-process state machine, for sanity checking that we do things in the
 * right order.
 *
 * Postmaster:
 *   INITIAL -> REQUESTING -> INITIALIZING -> DONE
 *
 * Backends in EXEC_BACKEND mode:
 *   INITIAL -> REQUESTING -> ATTACHING -> DONE
 *
 * Late request:
 *   DONE -> REQUESTING -> AFTER_STARTUP_ATTACH_OR_INIT -> DONE
 */
enum shmem_request_state
{
    /* Initial state */
    SRS_INITIAL,
 
    /*
     * When we start calling the shmem_request callbacks, we enter the
     * SRS_REQUESTING phase.  All ShmemRequestStruct calls happen in this
     * state.
     */
    SRS_REQUESTING,
 
    /*
     * Postmaster has finished all shmem requests, and is now initializing the
     * shared memory segment.  init_fn callbacks are called in this state.
     */
    SRS_INITIALIZING,
 
    /*
     * A postmaster child process is starting up.  attach_fn callbacks are
     * called in this state.
     */
    SRS_ATTACHING,
 
    /* An after-startup allocation or attachment is in progress */
    SRS_AFTER_STARTUP_ATTACH_OR_INIT,
 
    /* Normal state after shmem initialization / attachment */
    SRS_DONE,
};
static enum shmem_request_state shmem_request_state = SRS_INITIAL;
 
/*
 * This is the first data structure stored in the shared memory segment, at
 * the offset that PGShmemHeader->content_offset points to.  Allocations by
 * ShmemAlloc() are carved out of the space after this.
 *
 * For the base pointer and the total size of the shmem segment, we rely on
 * the PGShmemHeader.
 */
typedef struct ShmemAllocatorData
{
    Size        free_offset;    /* offset to first free space from ShmemBase */
 
    /* protects 'free_offset' */
    slock_t     shmem_lock;
 
    HASHHDR    *index;          /* location of ShmemIndex */
    size_t      index_size;     /* size of shmem region holding ShmemIndex */
    LWLock      index_lock;     /* protects ShmemIndex */
} ShmemAllocatorData;
 
#define ShmemIndexLock (&ShmemAllocator->index_lock)
 
static void *ShmemAllocRaw(Size size, Size alignment, Size *allocated_size);
 
/* shared memory global variables */
 
static PGShmemHeader *ShmemSegHdr;  /* shared mem segment header */
static void *ShmemBase;         /* start address of shared memory */
static void *ShmemEnd;          /* end+1 address of shared memory */
 
static ShmemAllocatorData *ShmemAllocator;
 
/*
 * ShmemIndex is a global directory of shmem areas, itself also stored in the
 * shared memory.
 */
static HTAB *ShmemIndex;
 
 /* max size of data structure string name */
#define SHMEM_INDEX_KEYSIZE      (48)
 
/*
 * # of additional entries to reserve in the shmem index table, for
 * allocations after postmaster startup.  (This is not a hard limit, the hash
 * table can grow larger than that if there is shared memory available)
 */
#define SHMEM_INDEX_ADDITIONAL_SIZE      (128)
 
/* this is a hash bucket in the shmem index table */
typedef struct
{
    char        key[SHMEM_INDEX_KEYSIZE];   /* string name */
    void       *location;       /* location in shared mem */
    Size        size;           /* # bytes requested for the structure */
    Size        allocated_size; /* # bytes actually allocated */
} ShmemIndexEnt;
 
/* To get reliable results for NUMA inquiry we need to "touch pages" once */
static bool firstNumaTouch = true;
 
static void CallShmemCallbacksAfterStartup(const ShmemCallbacks *callbacks);
static void InitShmemIndexEntry(ShmemRequest *request);
static bool AttachShmemIndexEntry(ShmemRequest *request, bool missing_ok);
 
Datum       pg_numa_available(PG_FUNCTION_ARGS);
 
/*
 *  ShmemRequestStruct() --- request a named shared memory area
 *
 * Subsystems call this to register their shared memory needs.  This is
 * usually done early in postmaster startup, before the shared memory segment
 * has been created, so that the size can be included in the estimate for
 * total amount of shared memory needed.  We set aside a small amount of
 * memory for allocations that happen later, for the benefit of non-preloaded
 * extensions, but that should not be relied upon.
 *
 * This does not yet allocate the memory, but merely registers the need for
 * it.  The actual allocation happens later in the postmaster startup
 * sequence.
 *
 * This must be called from a shmem_request callback function, registered with
 * RegisterShmemCallbacks().  This enforces a coding pattern that works the
 * same in normal Unix systems and with EXEC_BACKEND.  On Unix systems, the
 * shmem_request callbacks are called once, early in postmaster startup, and
 * the child processes inherit the struct descriptors and any other
 * per-process state from the postmaster.  In EXEC_BACKEND mode, shmem_request
 * callbacks are *also* called in each backend, at backend startup, to
 * re-establish the struct descriptors.  By calling the same function in both
 * cases, we ensure that all the shmem areas are registered the same way in
 * all processes.
 *
 * 'options' defines the name and size of the area, and any other optional
 * features.  Leave unused options as zeros.  The options are copied to
 * longer-lived memory, so it doesn't need to live after the
 * ShmemRequestStruct() call and can point to a local variable in the calling
 * function.  The 'name' must point to a long-lived string though, only the
 * pointer to it is copied.
 */
void
ShmemRequestStructWithOpts(const ShmemStructOpts *options)
{
    ShmemStructOpts *options_copy;
 
    options_copy = MemoryContextAlloc(TopMemoryContext,
                                      sizeof(ShmemStructOpts));
    memcpy(options_copy, options, sizeof(ShmemStructOpts));
 
    ShmemRequestInternal(options_copy, SHMEM_KIND_STRUCT);
}
 
/*
 * Internal workhorse of ShmemRequestStruct() and ShmemRequestHash().
 *
 * Note: Unlike in the public ShmemRequestStruct() and ShmemRequestHash()
 * functions, 'options' is *not* copied.  It must be allocated in
 * TopMemoryContext by the caller, and will be freed after the init/attach
 * callbacks have been called.  This allows ShmemRequestHash() to pass a
 * pointer to the extended ShmemHashOpts struct instead.
 */
void
ShmemRequestInternal(ShmemStructOpts *options, ShmemRequestKind kind)
{
    ShmemRequest *request;
 
    /* Check the options */
    if (options->name == NULL)
        elog(ERROR, "shared memory request is missing 'name' option");
 
    if (IsUnderPostmaster)
    {
        if (options->size <= 0 && options->size != SHMEM_ATTACH_UNKNOWN_SIZE)
            elog(ERROR, "invalid size %zd for shared memory request for \"%s\"",
                 options->size, options->name);
    }
    else
    {
        if (options->size == SHMEM_ATTACH_UNKNOWN_SIZE)
            elog(ERROR, "SHMEM_ATTACH_UNKNOWN_SIZE cannot be used during startup");
        if (options->size <= 0)
            elog(ERROR, "invalid size %zd for shared memory request for \"%s\"",
                 options->size, options->name);
    }
 
    if (options->alignment != 0 && pg_nextpower2_size_t(options->alignment) != options->alignment)
        elog(ERROR, "invalid alignment %zu for shared memory request for \"%s\"",
             options->alignment, options->name);
 
    /* Check that we're in the right state */
    if (shmem_request_state != SRS_REQUESTING)
        elog(ERROR, "ShmemRequestStruct can only be called from a shmem_request callback");
 
    /* Check that it's not already registered in this process */
    foreach_ptr(ShmemRequest, existing, pending_shmem_requests)
    {
        if (strcmp(existing->options->name, options->name) == 0)
            ereport(ERROR,
                    (errmsg("shared memory struct \"%s\" is already registered",
                            options->name)));
    }
 
    /* Request looks valid, remember it */
    request = palloc(sizeof(ShmemRequest));
    request->options = options;
    request->kind = kind;
    pending_shmem_requests = lappend(pending_shmem_requests, request);
}
 
/*
 *  ShmemGetRequestedSize() --- estimate the total size of all registered shared
 *                              memory structures.
 *
 * This is called at postmaster startup, before the shared memory segment has
 * been created.
 */
size_t
ShmemGetRequestedSize(void)
{
    size_t      size;
 
    /* memory needed for the ShmemIndex */
    size = hash_estimate_size(list_length(pending_shmem_requests) + SHMEM_INDEX_ADDITIONAL_SIZE,
                              sizeof(ShmemIndexEnt));
    size = CACHELINEALIGN(size);
 
    /* memory needed for all the requested areas */
    foreach_ptr(ShmemRequest, request, pending_shmem_requests)
    {
        size_t      alignment = request->options->alignment;
 
        /* pad the start address for alignment like ShmemAllocRaw() does */
        if (alignment < PG_CACHE_LINE_SIZE)
            alignment = PG_CACHE_LINE_SIZE;
        size = TYPEALIGN(alignment, size);
 
        size = add_size(size, request->options->size);
    }
 
    return size;
}
 
/*
 *  ShmemInitRequested() --- allocate and initialize requested shared memory
 *                            structures.
 *
 * This is called once at postmaster startup, after the shared memory segment
 * has been created.
 */
void
ShmemInitRequested(void)
{
    /* should be called only by the postmaster or a standalone backend */
    Assert(!IsUnderPostmaster);
    Assert(shmem_request_state == SRS_INITIALIZING);
 
    /*
     * Initialize the ShmemIndex entries and perform basic initialization of
     * all the requested memory areas.  There are no concurrent processes yet,
     * so no need for locking.
     */
    foreach_ptr(ShmemRequest, request, pending_shmem_requests)
    {
        InitShmemIndexEntry(request);
        pfree(request->options);
    }
    list_free_deep(pending_shmem_requests);
    pending_shmem_requests = NIL;
 
    /*
     * Call the subsystem-specific init callbacks to finish initialization of
     * all the areas.
     */
    foreach_ptr(const ShmemCallbacks, callbacks, registered_shmem_callbacks)
    {
        if (callbacks->init_fn)
            callbacks->init_fn(callbacks->opaque_arg);
    }
 
    shmem_request_state = SRS_DONE;
}
 
/*
 * Re-establish process private state related to shmem areas.
 *
 * This is called at backend startup in EXEC_BACKEND mode, in every backend.
 */
#ifdef EXEC_BACKEND
void
ShmemAttachRequested(void)
{
    ListCell   *lc;
 
    /* Must be initializing a (non-standalone) backend */
    Assert(IsUnderPostmaster);
    Assert(ShmemAllocator->index != NULL);
    Assert(shmem_request_state == SRS_REQUESTING);
    shmem_request_state = SRS_ATTACHING;
 
    LWLockAcquire(ShmemIndexLock, LW_SHARED);
 
    /*
     * Attach to all the requested memory areas.
     */
    foreach_ptr(ShmemRequest, request, pending_shmem_requests)
    {
        AttachShmemIndexEntry(request, false);
        pfree(request->options);
    }
    list_free_deep(pending_shmem_requests);
    pending_shmem_requests = NIL;
 
    /* Call attach callbacks */
    foreach(lc, registered_shmem_callbacks)
    {
        const ShmemCallbacks *callbacks = (const ShmemCallbacks *) lfirst(lc);
 
        if (callbacks->attach_fn)
            callbacks->attach_fn(callbacks->opaque_arg);
    }
 
    LWLockRelease(ShmemIndexLock);
 
    shmem_request_state = SRS_DONE;
}
#endif
 
/*
 * Insert requested shmem area into the shared memory index and initialize it.
 *
 * Note that this only does performs basic initialization depending on
 * ShmemRequestKind, like setting the global pointer variable to the area for
 * SHMEM_KIND_STRUCT or setting up the backend-private HTAB control struct.
 * This does *not* call the subsystem-specific init callbacks.  That's done
 * later after all the shmem areas have been initialized or attached to.
 */
static void
InitShmemIndexEntry(ShmemRequest *request)
{
    const char *name = request->options->name;
    ShmemIndexEnt *index_entry;
    bool        found;
    size_t      allocated_size;
    void       *structPtr;
 
    /* look it up in the shmem index */
    index_entry = (ShmemIndexEnt *)
        hash_search(ShmemIndex, name, HASH_ENTER_NULL, &found);
    if (found)
        elog(ERROR, "shared memory struct \"%s\" is already initialized", name);
    if (!index_entry)
    {
        /* tried to add it to the hash table, but there was no space */
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("could not create ShmemIndex entry for data structure \"%s\"",
                        name)));
    }
 
    /*
     * We inserted the entry to the shared memory index.  Allocate requested
     * amount of shared memory for it, and initialize the index entry.
     */
    structPtr = ShmemAllocRaw(request->options->size,
                              request->options->alignment,
                              &allocated_size);
    if (structPtr == NULL)
    {
        /* out of memory; remove the failed ShmemIndex entry */
        hash_search(ShmemIndex, name, HASH_REMOVE, NULL);
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("not enough shared memory for data structure"
                        " \"%s\" (%zd bytes requested)",
                        name, request->options->size)));
    }
    index_entry->size = request->options->size;
    index_entry->allocated_size = allocated_size;
    index_entry->location = structPtr;
 
    /* Initialize depending on the kind of shmem area it is */
    switch (request->kind)
    {
        case SHMEM_KIND_STRUCT:
            if (request->options->ptr)
                *(request->options->ptr) = index_entry->location;
            break;
        case SHMEM_KIND_HASH:
            shmem_hash_init(structPtr, request->options);
            break;
        case SHMEM_KIND_SLRU:
            shmem_slru_init(structPtr, request->options);
            break;
    }
}
 
/*
 * Look up a named shmem area in the shared memory index and attach to it.
 *
 * Note that this only performs the basic attachment actions depending on
 * ShmemRequestKind, like setting the global pointer variable to the area for
 * SHMEM_KIND_STRUCT or setting up the backend-private HTAB control struct.
 * This does *not* call the subsystem-specific attach callbacks.  That's done
 * later after all the shmem areas have been initialized or attached to.
 */
static bool
AttachShmemIndexEntry(ShmemRequest *request, bool missing_ok)
{
    const char *name = request->options->name;
    ShmemIndexEnt *index_entry;
 
    /* Look it up in the shmem index */
    index_entry = (ShmemIndexEnt *)
        hash_search(ShmemIndex, name, HASH_FIND, NULL);
    if (!index_entry)
    {
        if (!missing_ok)
            ereport(ERROR,
                    (errmsg("could not find ShmemIndex entry for data structure \"%s\"",
                            request->options->name)));
        return false;
    }
 
    /* Check that the size in the index matches the request */
    if (index_entry->size != request->options->size &&
        request->options->size != SHMEM_ATTACH_UNKNOWN_SIZE)
    {
        ereport(ERROR,
                (errmsg("shared memory struct \"%s\" was created with"
                        " different size: existing %zu, requested %zd",
                        name, index_entry->size, request->options->size)));
    }
 
    /*
     * Re-establish the caller's pointer variable, or do other actions to
     * attach depending on the kind of shmem area it is.
     */
    switch (request->kind)
    {
        case SHMEM_KIND_STRUCT:
            if (request->options->ptr)
                *(request->options->ptr) = index_entry->location;
            break;
        case SHMEM_KIND_HASH:
            shmem_hash_attach(index_entry->location, request->options);
            break;
        case SHMEM_KIND_SLRU:
            shmem_slru_attach(index_entry->location, request->options);
            break;
    }
 
    return true;
}
 
/*
 *  InitShmemAllocator() --- set up basic pointers to shared memory.
 *
 * Called at postmaster or stand-alone backend startup, to initialize the
 * allocator's data structure in the shared memory segment.  In EXEC_BACKEND,
 * this is also called at backend startup, to set up pointers to the
 * already-initialized data structure.
 */
void
InitShmemAllocator(PGShmemHeader *seghdr)
{
    Size        offset;
    int64       hash_nelems;
    HASHCTL     info;
    int         hash_flags;
 
#ifndef EXEC_BACKEND
    Assert(!IsUnderPostmaster);
#endif
    Assert(seghdr != NULL);
 
    if (IsUnderPostmaster)
    {
        Assert(shmem_request_state == SRS_INITIAL);
    }
    else
    {
        Assert(shmem_request_state == SRS_REQUESTING);
        shmem_request_state = SRS_INITIALIZING;
    }
 
    /*
     * We assume the pointer and offset are MAXALIGN.  Not a hard requirement,
     * but it's true today and keeps the math below simpler.
     */
    Assert(seghdr == (void *) MAXALIGN(seghdr));
    Assert(seghdr->content_offset == MAXALIGN(seghdr->content_offset));
 
    /*
     * Allocations after this point should go through ShmemAlloc, which
     * expects to allocate everything on cache line boundaries.  Make sure the
     * first allocation begins on a cache line boundary.
     */
    offset = CACHELINEALIGN(seghdr->content_offset + sizeof(ShmemAllocatorData));
    if (offset > seghdr->totalsize)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory (%zu bytes requested)",
                        offset)));
 
    /*
     * In postmaster or stand-alone backend, initialize the shared memory
     * allocator so that we can allocate shared memory for ShmemIndex using
     * ShmemAlloc().  In a regular backend just set up the pointers required
     * by ShmemAlloc().
     */
    ShmemAllocator = (ShmemAllocatorData *) ((char *) seghdr + seghdr->content_offset);
    if (!IsUnderPostmaster)
    {
        SpinLockInit(&ShmemAllocator->shmem_lock);
        ShmemAllocator->free_offset = offset;
        LWLockInitialize(&ShmemAllocator->index_lock, LWTRANCHE_SHMEM_INDEX);
    }
 
    ShmemSegHdr = seghdr;
    ShmemBase = seghdr;
    ShmemEnd = (char *) ShmemBase + seghdr->totalsize;
 
    /*
     * Create (or attach to) the shared memory index of shmem areas.
     *
     * This is the same initialization as ShmemInitHash() does, but we cannot
     * use ShmemInitHash() here because it relies on ShmemIndex being already
     * initialized.
     */
    hash_nelems = list_length(pending_shmem_requests) + SHMEM_INDEX_ADDITIONAL_SIZE;
 
    info.keysize = SHMEM_INDEX_KEYSIZE;
    info.entrysize = sizeof(ShmemIndexEnt);
    hash_flags = HASH_ELEM | HASH_STRINGS | HASH_FIXED_SIZE;
 
    if (!IsUnderPostmaster)
    {
        ShmemAllocator->index_size = hash_estimate_size(hash_nelems, info.entrysize);
        ShmemAllocator->index = (HASHHDR *) ShmemAlloc(ShmemAllocator->index_size);
    }
    ShmemIndex = shmem_hash_create(ShmemAllocator->index,
                                   ShmemAllocator->index_size,
                                   IsUnderPostmaster,
                                   "ShmemIndex", hash_nelems,
                                   &info, hash_flags);
    Assert(ShmemIndex != NULL);
 
    /*
     * Add an entry for ShmemIndex itself into ShmemIndex, so that it's
     * visible in the pg_shmem_allocations view
     */
    if (!IsUnderPostmaster)
    {
        bool        found;
        ShmemIndexEnt *result = (ShmemIndexEnt *)
            hash_search(ShmemIndex, "ShmemIndex", HASH_ENTER, &found);
 
        Assert(!found);
        result->size = ShmemAllocator->index_size;
        result->allocated_size = ShmemAllocator->index_size;
        result->location = ShmemAllocator->index;
    }
}
 
/*
 * Reset state on postmaster crash restart.
 */
void
ResetShmemAllocator(void)
{
    Assert(!IsUnderPostmaster);
    shmem_request_state = SRS_INITIAL;
 
    pending_shmem_requests = NIL;
 
    /*
     * Note that we don't clear the registered callbacks.  We will need to
     * call them again as we restart
     */
}
 
/*
 * ShmemAlloc -- allocate max-aligned chunk from shared memory
 *
 * Throws error if request cannot be satisfied.
 *
 * Assumes ShmemSegHdr is initialized.
 */
void *
ShmemAlloc(Size size)
{
    void       *newSpace;
    Size        allocated_size;
 
    newSpace = ShmemAllocRaw(size, 0, &allocated_size);
    if (!newSpace)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory (%zu bytes requested)",
                        size)));
    return newSpace;
}
 
/*
 * ShmemAllocNoError -- allocate max-aligned chunk from shared memory
 *
 * As ShmemAlloc, but returns NULL if out of space, rather than erroring.
 */
void *
ShmemAllocNoError(Size size)
{
    Size        allocated_size;
 
    return ShmemAllocRaw(size, 0, &allocated_size);
}
 
/*
 * ShmemAllocRaw -- allocate align chunk and return allocated size
 *
 * Also sets *allocated_size to the number of bytes allocated, which will
 * be equal to the number requested plus any padding we choose to add.
 */
static void *
ShmemAllocRaw(Size size, Size alignment, Size *allocated_size)
{
    Size        rawStart;
    Size        newStart;
    Size        newFree;
    void       *newSpace;
 
    /*
     * Ensure all space is adequately aligned.  We used to only MAXALIGN this
     * space but experience has proved that on modern systems that is not good
     * enough.  Many parts of the system are very sensitive to critical data
     * structures getting split across cache line boundaries.  To avoid that,
     * attempt to align the beginning of the allocation to a cache line
     * boundary.  The calling code will still need to be careful about how it
     * uses the allocated space - e.g. by padding each element in an array of
     * structures out to a power-of-two size - but without this, even that
     * won't be sufficient.
     */
    if (alignment < PG_CACHE_LINE_SIZE)
        alignment = PG_CACHE_LINE_SIZE;
 
    Assert(ShmemSegHdr != NULL);
 
    SpinLockAcquire(&ShmemAllocator->shmem_lock);
 
    rawStart = ShmemAllocator->free_offset;
    newStart = TYPEALIGN(alignment, rawStart);
 
    newFree = newStart + size;
    if (newFree <= ShmemSegHdr->totalsize)
    {
        newSpace = (char *) ShmemBase + newStart;
        ShmemAllocator->free_offset = newFree;
    }
    else
        newSpace = NULL;
 
    SpinLockRelease(&ShmemAllocator->shmem_lock);
 
    /* note this assert is okay with newSpace == NULL */
    Assert(newSpace == (void *) TYPEALIGN(alignment, newSpace));
 
    *allocated_size = newFree - rawStart;
    return newSpace;
}
 
/*
 * ShmemAddrIsValid -- test if an address refers to shared memory
 *
 * Returns true if the pointer points within the shared memory segment.
 */
bool
ShmemAddrIsValid(const void *addr)
{
    return (addr >= ShmemBase) && (addr < ShmemEnd);
}
 
/*
 * Register callbacks that define a shared memory area (or multiple areas).
 *
 * The system will call the callbacks at different stages of postmaster or
 * backend startup, to allocate and initialize the area.
 *
 * This is normally called early during postmaster startup, but if the
 * SHMEM_CALLBACKS_ALLOW_AFTER_STARTUP is set, this can also be used after
 * startup, although after startup there's no guarantee that there's enough
 * shared memory available.  When called after startup, this immediately calls
 * the right callbacks depending on whether another backend had already
 * initialized the area.
 *
 * Note: In EXEC_BACKEND mode, this needs to be called in every backend
 * process.  That's needed because we cannot pass down the callback function
 * pointers from the postmaster process, because different processes may have
 * loaded libraries to different addresses.
 */
void
RegisterShmemCallbacks(const ShmemCallbacks *callbacks)
{
    if (shmem_request_state == SRS_DONE && IsUnderPostmaster)
    {
        /*
         * After-startup initialization or attachment.  Call the appropriate
         * callbacks immediately.
         */
        if ((callbacks->flags & SHMEM_CALLBACKS_ALLOW_AFTER_STARTUP) == 0)
            elog(ERROR, "cannot request shared memory at this time");
 
        CallShmemCallbacksAfterStartup(callbacks);
    }
    else
    {
        /* Remember the callbacks for later */
        registered_shmem_callbacks = lappend(registered_shmem_callbacks,
                                             (void *) callbacks);
    }
}
 
/*
 * Register a shmem area (or multiple areas) after startup.
 */
static void
CallShmemCallbacksAfterStartup(const ShmemCallbacks *callbacks)
{
    bool        found_any;
    bool        notfound_any;
 
    Assert(shmem_request_state == SRS_DONE);
    shmem_request_state = SRS_REQUESTING;
 
    /*
     * Call the request callback first.  The callback makes ShmemRequest*()
     * calls for each shmem area, adding them to pending_shmem_requests.
     */
    Assert(pending_shmem_requests == NIL);
    if (callbacks->request_fn)
        callbacks->request_fn(callbacks->opaque_arg);
    shmem_request_state = SRS_AFTER_STARTUP_ATTACH_OR_INIT;
 
    if (pending_shmem_requests == NIL)
    {
        shmem_request_state = SRS_DONE;
        return;
    }
 
    /* Hold ShmemIndexLock while we allocate all the shmem entries */
    LWLockAcquire(ShmemIndexLock, LW_EXCLUSIVE);
 
    /*
     * Check if the requested shared memory areas have already been
     * initialized.  We assume all the areas requested by the request callback
     * to form a coherent unit such that they're all already initialized or
     * none.  Otherwise it would be ambiguous which callback, init or attach,
     * to callback afterwards.
     */
    found_any = notfound_any = false;
    foreach_ptr(ShmemRequest, request, pending_shmem_requests)
    {
        if (hash_search(ShmemIndex, request->options->name, HASH_FIND, NULL))
            found_any = true;
        else
            notfound_any = true;
    }
    if (found_any && notfound_any)
        elog(ERROR, "found some but not all");
 
    /*
     * Allocate or attach all the shmem areas requested by the request_fn
     * callback.
     */
    foreach_ptr(ShmemRequest, request, pending_shmem_requests)
    {
        if (found_any)
            AttachShmemIndexEntry(request, false);
        else
            InitShmemIndexEntry(request);
 
        pfree(request->options);
    }
    list_free_deep(pending_shmem_requests);
    pending_shmem_requests = NIL;
 
    /* Finish by calling the appropriate subsystem-specific callback */
    if (found_any)
    {
        if (callbacks->attach_fn)
            callbacks->attach_fn(callbacks->opaque_arg);
    }
    else
    {
        if (callbacks->init_fn)
            callbacks->init_fn(callbacks->opaque_arg);
    }
 
    LWLockRelease(ShmemIndexLock);
    shmem_request_state = SRS_DONE;
}
 
/*
 * Call all shmem request callbacks.
 */
void
ShmemCallRequestCallbacks(void)
{
    ListCell   *lc;
 
    Assert(shmem_request_state == SRS_INITIAL);
    shmem_request_state = SRS_REQUESTING;
 
    foreach(lc, registered_shmem_callbacks)
    {
        const ShmemCallbacks *callbacks = (const ShmemCallbacks *) lfirst(lc);
 
        if (callbacks->request_fn)
            callbacks->request_fn(callbacks->opaque_arg);
    }
}
 
/*
 * ShmemInitStruct -- Create/attach to a structure in shared memory.
 *
 *      This is called during initialization to find or allocate
 *      a data structure in shared memory.  If no other process
 *      has created the structure, this routine allocates space
 *      for it.  If it exists already, a pointer to the existing
 *      structure is returned.
 *
 *  Returns: pointer to the object.  *foundPtr is set true if the object was
 *      already in the shmem index (hence, already initialized).
 *
 * Note: This is a legacy interface, kept for backwards compatibility with
 * extensions.  Use ShmemRequestStruct() in new code!
 */
void *
ShmemInitStruct(const char *name, Size size, bool *foundPtr)
{
    void       *ptr = NULL;
    ShmemStructOpts options = {
        .name = name,
        .size = size,
        .ptr = &ptr,
    };
    ShmemRequest request = {&options, SHMEM_KIND_STRUCT};
 
    Assert(shmem_request_state == SRS_DONE ||
           shmem_request_state == SRS_INITIALIZING ||
           shmem_request_state == SRS_REQUESTING);
 
    LWLockAcquire(ShmemIndexLock, LW_EXCLUSIVE);
 
    /*
     * During postmaster startup, look up the existing entry if any.
     */
    *foundPtr = false;
    if (IsUnderPostmaster)
        *foundPtr = AttachShmemIndexEntry(&request, true);
 
    /* Initialize it if not found */
    if (!*foundPtr)
        InitShmemIndexEntry(&request);
 
    LWLockRelease(ShmemIndexLock);
 
    Assert(ptr != NULL);
    return ptr;
}
 
/* SQL SRF showing allocated shared memory */
Datum
pg_get_shmem_allocations(PG_FUNCTION_ARGS)
{
#define PG_GET_SHMEM_SIZES_COLS 4
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    HASH_SEQ_STATUS hstat;
    ShmemIndexEnt *ent;
    Size        named_allocated = 0;
    Datum       values[PG_GET_SHMEM_SIZES_COLS];
    bool        nulls[PG_GET_SHMEM_SIZES_COLS];
 
    InitMaterializedSRF(fcinfo, 0);
 
    LWLockAcquire(ShmemIndexLock, LW_SHARED);
 
    hash_seq_init(&hstat, ShmemIndex);
 
    /* output all allocated entries */
    memset(nulls, 0, sizeof(nulls));
    while ((ent = (ShmemIndexEnt *) hash_seq_search(&hstat)) != NULL)
    {
        values[0] = CStringGetTextDatum(ent->key);
        values[1] = Int64GetDatum((char *) ent->location - (char *) ShmemSegHdr);
        values[2] = Int64GetDatum(ent->size);
        values[3] = Int64GetDatum(ent->allocated_size);
        named_allocated += ent->allocated_size;
 
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
                             values, nulls);
    }
 
    /* output shared memory allocated but not counted via the shmem index */
    values[0] = CStringGetTextDatum("<anonymous>");
    nulls[1] = true;
    values[2] = Int64GetDatum(ShmemAllocator->free_offset - named_allocated);
    values[3] = values[2];
    tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
 
    /* output as-of-yet unused shared memory */
    nulls[0] = true;
    values[1] = Int64GetDatum(ShmemAllocator->free_offset);
    nulls[1] = false;
    values[2] = Int64GetDatum(ShmemSegHdr->totalsize - ShmemAllocator->free_offset);
    values[3] = values[2];
    tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
 
    LWLockRelease(ShmemIndexLock);
 
    return (Datum) 0;
}
 
/*
 * SQL SRF showing NUMA memory nodes for allocated shared memory
 *
 * Compared to pg_get_shmem_allocations(), this function does not return
 * information about shared anonymous allocations and unused shared memory.
 */
Datum
pg_get_shmem_allocations_numa(PG_FUNCTION_ARGS)
{
#define PG_GET_SHMEM_NUMA_SIZES_COLS 3
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    HASH_SEQ_STATUS hstat;
    ShmemIndexEnt *ent;
    Datum       values[PG_GET_SHMEM_NUMA_SIZES_COLS];
    bool        nulls[PG_GET_SHMEM_NUMA_SIZES_COLS];
    Size        os_page_size;
    void      **page_ptrs;
    int        *pages_status;
    uint64      shm_total_page_count,
                shm_ent_page_count,
                max_nodes;
    Size       *nodes;
 
    if (pg_numa_init() == -1)
        elog(ERROR, "libnuma initialization failed or NUMA is not supported on this platform");
 
    InitMaterializedSRF(fcinfo, 0);
 
    max_nodes = pg_numa_get_max_node();
    nodes = palloc_array(Size, max_nodes + 2);
 
    /*
     * Shared memory allocations can vary in size and may not align with OS
     * memory page boundaries, while NUMA queries work on pages.
     *
     * To correctly map each allocation to NUMA nodes, we need to: 1.
     * Determine the OS memory page size. 2. Align each allocation's start/end
     * addresses to page boundaries. 3. Query NUMA node information for all
     * pages spanning the allocation.
     */
    os_page_size = pg_get_shmem_pagesize();
 
    /*
     * Allocate memory for page pointers and status based on total shared
     * memory size. This simplified approach allocates enough space for all
     * pages in shared memory rather than calculating the exact requirements
     * for each segment.
     *
     * Add 1, because we don't know how exactly the segments align to OS
     * pages, so the allocation might use one more memory page. In practice
     * this is not very likely, and moreover we have more entries, each of
     * them using only fraction of the total pages.
     */
    shm_total_page_count = (ShmemSegHdr->totalsize / os_page_size) + 1;
    page_ptrs = palloc0_array(void *, shm_total_page_count);
    pages_status = palloc_array(int, shm_total_page_count);
 
    if (firstNumaTouch)
        elog(DEBUG1, "NUMA: page-faulting shared memory segments for proper NUMA readouts");
 
    LWLockAcquire(ShmemIndexLock, LW_SHARED);
 
    hash_seq_init(&hstat, ShmemIndex);
 
    /* output all allocated entries */
    while ((ent = (ShmemIndexEnt *) hash_seq_search(&hstat)) != NULL)
    {
        int         i;
        char       *startptr,
                   *endptr;
        Size        total_len;
 
        /*
         * Calculate the range of OS pages used by this segment. The segment
         * may start / end half-way through a page, we want to count these
         * pages too. So we align the start/end pointers down/up, and then
         * calculate the number of pages from that.
         */
        startptr = (char *) TYPEALIGN_DOWN(os_page_size, ent->location);
        endptr = (char *) TYPEALIGN(os_page_size,
                                    (char *) ent->location + ent->allocated_size);
        total_len = (endptr - startptr);
 
        shm_ent_page_count = total_len / os_page_size;
 
        /*
         * If we ever get 0xff (-1) back from kernel inquiry, then we probably
         * have a bug in mapping buffers to OS pages.
         */
        memset(pages_status, 0xff, sizeof(int) * shm_ent_page_count);
 
        /*
         * Setup page_ptrs[] with pointers to all OS pages for this segment,
         * and get the NUMA status using pg_numa_query_pages.
         *
         * In order to get reliable results we also need to touch memory
         * pages, so that inquiry about NUMA memory node doesn't return -2
         * (ENOENT, which indicates unmapped/unallocated pages).
         */
        for (i = 0; i < shm_ent_page_count; i++)
        {
            page_ptrs[i] = startptr + (i * os_page_size);
 
            if (firstNumaTouch)
                pg_numa_touch_mem_if_required(page_ptrs[i]);
 
            CHECK_FOR_INTERRUPTS();
        }
 
        if (pg_numa_query_pages(0, shm_ent_page_count, page_ptrs, pages_status) == -1)
            elog(ERROR, "failed NUMA pages inquiry status: %m");
 
        /* Count number of NUMA nodes used for this shared memory entry */
        memset(nodes, 0, sizeof(Size) * (max_nodes + 2));
 
        for (i = 0; i < shm_ent_page_count; i++)
        {
            int         s = pages_status[i];
 
            /* Ensure we are adding only valid index to the array */
            if (s >= 0 && s <= max_nodes)
            {
                /* valid NUMA node */
                nodes[s]++;
                continue;
            }
            else if (s == -2)
            {
                /* -2 means ENOENT (e.g. page was moved to swap) */
                nodes[max_nodes + 1]++;
                continue;
            }
 
            elog(ERROR, "invalid NUMA node id outside of allowed range "
                 "[0, " UINT64_FORMAT "]: %d", max_nodes, s);
        }
 
        /* no NULLs for regular nodes */
        memset(nulls, 0, sizeof(nulls));
 
        /*
         * Add one entry for each NUMA node, including those without allocated
         * memory for this segment.
         */
        for (i = 0; i <= max_nodes; i++)
        {
            values[0] = CStringGetTextDatum(ent->key);
            values[1] = Int32GetDatum(i);
            values[2] = Int64GetDatum(nodes[i] * os_page_size);
 
            tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
                                 values, nulls);
        }
 
        /* The last entry is used for pages without a NUMA node. */
        nulls[1] = true;
        values[0] = CStringGetTextDatum(ent->key);
        values[2] = Int64GetDatum(nodes[max_nodes + 1] * os_page_size);
 
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
                             values, nulls);
    }
 
    LWLockRelease(ShmemIndexLock);
    firstNumaTouch = false;
 
    return (Datum) 0;
}
 
/*
 * Determine the memory page size used for the shared memory segment.
 *
 * If the shared segment was allocated using huge pages, returns the size of
 * a huge page. Otherwise returns the size of regular memory page.
 *
 * This should be used only after the server is started.
 */
Size
pg_get_shmem_pagesize(void)
{
    Size        os_page_size;
#ifdef WIN32
    SYSTEM_INFO sysinfo;
 
    GetSystemInfo(&sysinfo);
    os_page_size = sysinfo.dwPageSize;
#else
    os_page_size = sysconf(_SC_PAGESIZE);
#endif
 
    Assert(IsUnderPostmaster);
    Assert(huge_pages_status != HUGE_PAGES_UNKNOWN);
 
    if (huge_pages_status == HUGE_PAGES_ON)
        GetHugePageSize(&os_page_size, NULL);
 
    return os_page_size;
}
 
Datum
pg_numa_available(PG_FUNCTION_ARGS)
{
    PG_RETURN_BOOL(pg_numa_init() != -1);
}