shmem.c

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/*-------------------------------------------------------------------------
 *
 * shmem.c
 *    create shared memory and initialize shared memory data structures.
 *
 * Portions Copyright (c) 1996-2019, 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.
 *
 * NOTES:
 *      (a) There are three kinds of shared memory data structures
 *  available to POSTGRES: fixed-size structures, queues and hash
 *  tables.  Fixed-size structures contain things like global variables
 *  for a module and should never be allocated after the shared memory
 *  initialization phase.  Hash tables have a fixed maximum size, but
 *  their actual size can vary dynamically.  When entries are added
 *  to the table, more space is allocated.  Queues link data structures
 *  that have been allocated either within fixed-size structures or as hash
 *  buckets.  Each shared data structure has a string name to identify
 *  it (assigned in the module that declares it).
 *
 *      (b) During initialization, each module looks for its
 *  shared data structures in a hash table called the "Shmem Index".
 *  If the data structure is not present, the caller can allocate
 *  a new one and initialize it.  If the data structure is present,
 *  the caller "attaches" to the structure by initializing a pointer
 *  in the local address space.
 *      The shmem index has two purposes: first, it gives us
 *  a simple model of how the world looks when a backend process
 *  initializes.  If something is present in the shmem index,
 *  it is initialized.  If it is not, it is uninitialized.  Second,
 *  the shmem index allows us to allocate shared memory on demand
 *  instead of trying to preallocate structures and hard-wire the
 *  sizes and locations in header files.  If you are using a lot
 *  of shared memory in a lot of different places (and changing
 *  things during development), this is important.
 *
 *      (c) 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, new backends have to set up their local
 *  pointers using the method described in (b) above.
 *
 *      (d) memory allocation model: shared memory 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.  However,
 *  if one hash table grows very large and then shrinks, its space
 *  cannot be redistributed to other tables.  We could build a simple
 *  hash bucket garbage collector if need be.  Right now, it seems
 *  unnecessary.
 */

#include "postgres.h"

#include "access/transam.h"
#include "miscadmin.h"
#include "storage/lwlock.h"
#include "storage/pg_shmem.h"
#include "storage/shmem.h"
#include "storage/spin.h"


/* 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 */

slock_t    *ShmemLock;          /* spinlock for shared memory and LWLock
                                 * allocation */

static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */


/*
 *  InitShmemAccess() --- set up basic pointers to shared memory.
 *
 * Note: the argument should be declared "PGShmemHeader *seghdr",
 * but we use void to avoid having to include ipc.h in shmem.h.
 */
void
InitShmemAccess(void *seghdr)
{
    PGShmemHeader *shmhdr = (PGShmemHeader *) seghdr;

    ShmemSegHdr = shmhdr;
    ShmemBase = (void *) shmhdr;
    ShmemEnd = (char *) ShmemBase + shmhdr->totalsize;
}

/*
 *  InitShmemAllocation() --- set up shared-memory space allocation.
 *
 * This should be called only in the postmaster or a standalone backend.
 */
void
InitShmemAllocation(void)
{
    PGShmemHeader *shmhdr = ShmemSegHdr;
    char       *aligned;

    Assert(shmhdr != NULL);

    /*
     * Initialize the spinlock used by ShmemAlloc.  We must use
     * ShmemAllocUnlocked, since obviously ShmemAlloc can't be called yet.
     */
    ShmemLock = (slock_t *) ShmemAllocUnlocked(sizeof(slock_t));

    SpinLockInit(ShmemLock);

    /*
     * 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.
     */
    aligned = (char *)
        (CACHELINEALIGN((((char *) shmhdr) + shmhdr->freeoffset)));
    shmhdr->freeoffset = aligned - (char *) shmhdr;

    /* ShmemIndex can't be set up yet (need LWLocks first) */
    shmhdr->index = NULL;
    ShmemIndex = (HTAB *) NULL;

    /*
     * Initialize ShmemVariableCache for transaction manager. (This doesn't
     * really belong here, but not worth moving.)
     */
    ShmemVariableCache = (VariableCache)
        ShmemAlloc(sizeof(*ShmemVariableCache));
    memset(ShmemVariableCache, 0, sizeof(*ShmemVariableCache));
}

/*
 * ShmemAlloc -- allocate max-aligned chunk from shared memory
 *
 * Throws error if request cannot be satisfied.
 *
 * Assumes ShmemLock and ShmemSegHdr are initialized.
 */
void *
ShmemAlloc(Size size)
{
    void       *newSpace;

    newSpace = ShmemAllocNoError(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        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.
     */
    size = CACHELINEALIGN(size);

    Assert(ShmemSegHdr != NULL);

    SpinLockAcquire(ShmemLock);

    newStart = ShmemSegHdr->freeoffset;

    newFree = newStart + size;
    if (newFree <= ShmemSegHdr->totalsize)
    {
        newSpace = (void *) ((char *) ShmemBase + newStart);
        ShmemSegHdr->freeoffset = newFree;
    }
    else
        newSpace = NULL;

    SpinLockRelease(ShmemLock);

    /* note this assert is okay with newSpace == NULL */
    Assert(newSpace == (void *) CACHELINEALIGN(newSpace));

    return newSpace;
}

/*
 * ShmemAllocUnlocked -- allocate max-aligned chunk from shared memory
 *
 * Allocate space without locking ShmemLock.  This should be used for,
 * and only for, allocations that must happen before ShmemLock is ready.
 *
 * We consider maxalign, rather than cachealign, sufficient here.
 */
void *
ShmemAllocUnlocked(Size size)
{
    Size        newStart;
    Size        newFree;
    void       *newSpace;

    /*
     * Ensure allocated space is adequately aligned.
     */
    size = MAXALIGN(size);

    Assert(ShmemSegHdr != NULL);

    newStart = ShmemSegHdr->freeoffset;

    newFree = newStart + size;
    if (newFree > ShmemSegHdr->totalsize)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory (%zu bytes requested)",
                        size)));
    ShmemSegHdr->freeoffset = newFree;

    newSpace = (void *) ((char *) ShmemBase + newStart);

    Assert(newSpace == (void *) MAXALIGN(newSpace));

    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);
}

/*
 *  InitShmemIndex() --- set up or attach to shmem index table.
 */
void
InitShmemIndex(void)
{
    HASHCTL     info;
    int         hash_flags;

    /*
     * Create the shared memory shmem index.
     *
     * Since ShmemInitHash calls ShmemInitStruct, which expects the ShmemIndex
     * hashtable to exist already, we have a bit of a circularity problem in
     * initializing the ShmemIndex itself.  The special "ShmemIndex" hash
     * table name will tell ShmemInitStruct to fake it.
     */
    info.keysize = SHMEM_INDEX_KEYSIZE;
    info.entrysize = sizeof(ShmemIndexEnt);
    hash_flags = HASH_ELEM;

    ShmemIndex = ShmemInitHash("ShmemIndex",
                               SHMEM_INDEX_SIZE, SHMEM_INDEX_SIZE,
                               &info, hash_flags);
}

/*
 * ShmemInitHash -- Create and initialize, or attach to, a
 *      shared memory hash table.
 *
 * We assume caller is doing some kind of synchronization
 * so that two processes don't try to create/initialize the same
 * table at once.  (In practice, all creations are done in the postmaster
 * process; child processes should always be attaching to existing tables.)
 *
 * max_size is the estimated maximum number of hashtable entries.  This is
 * not a hard limit, but the access efficiency will degrade if it is
 * exceeded substantially (since it's used to compute directory size and
 * the hash table buckets will get overfull).
 *
 * init_size is the number of hashtable entries to preallocate.  For a table
 * whose maximum size is certain, this should be equal to max_size; that
 * ensures that no run-time out-of-shared-memory failures can occur.
 *
 * Note: before Postgres 9.0, this function returned NULL for some failure
 * cases.  Now, it always throws error instead, so callers need not check
 * for NULL.
 */
HTAB *
ShmemInitHash(const char *name,     /* table string name for shmem index */
              long init_size,   /* initial table size */
              long max_size,    /* max size of the table */
              HASHCTL *infoP,   /* info about key and bucket size */
              int hash_flags)   /* info about infoP */
{
    bool        found;
    void       *location;

    /*
     * Hash tables allocated in shared memory have a fixed directory; it can't
     * grow or other backends wouldn't be able to find it. So, make sure we
     * make it big enough to start with.
     *
     * The shared memory allocator must be specified too.
     */
    infoP->dsize = infoP->max_dsize = hash_select_dirsize(max_size);
    infoP->alloc = ShmemAllocNoError;
    hash_flags |= HASH_SHARED_MEM | HASH_ALLOC | HASH_DIRSIZE;

    /* look it up in the shmem index */
    location = ShmemInitStruct(name,
                               hash_get_shared_size(infoP, hash_flags),
                               &found);

    /*
     * if it already exists, attach to it rather than allocate and initialize
     * new space
     */
    if (found)
        hash_flags |= HASH_ATTACH;

    /* Pass location of hashtable header to hash_create */
    infoP->hctl = (HASHHDR *) location;

    return hash_create(name, init_size, infoP, hash_flags);
}

/*
 * 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: before Postgres 9.0, this function returned NULL for some failure
 *  cases.  Now, it always throws error instead, so callers need not check
 *  for NULL.
 */
void *
ShmemInitStruct(const char *name, Size size, bool *foundPtr)
{
    ShmemIndexEnt *result;
    void       *structPtr;

    LWLockAcquire(ShmemIndexLock, LW_EXCLUSIVE);

    if (!ShmemIndex)
    {
        PGShmemHeader *shmemseghdr = ShmemSegHdr;

        /* Must be trying to create/attach to ShmemIndex itself */
        Assert(strcmp(name, "ShmemIndex") == 0);

        if (IsUnderPostmaster)
        {
            /* Must be initializing a (non-standalone) backend */
            Assert(shmemseghdr->index != NULL);
            structPtr = shmemseghdr->index;
            *foundPtr = true;
        }
        else
        {
            /*
             * If the shmem index doesn't exist, we are bootstrapping: we must
             * be trying to init the shmem index itself.
             *
             * Notice that the ShmemIndexLock is released before the shmem
             * index has been initialized.  This should be OK because no other
             * process can be accessing shared memory yet.
             */
            Assert(shmemseghdr->index == NULL);
            structPtr = ShmemAlloc(size);
            shmemseghdr->index = structPtr;
            *foundPtr = false;
        }
        LWLockRelease(ShmemIndexLock);
        return structPtr;
    }

    /* look it up in the shmem index */
    result = (ShmemIndexEnt *)
        hash_search(ShmemIndex, name, HASH_ENTER_NULL, foundPtr);

    if (!result)
    {
        LWLockRelease(ShmemIndexLock);
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("could not create ShmemIndex entry for data structure \"%s\"",
                        name)));
    }

    if (*foundPtr)
    {
        /*
         * Structure is in the shmem index so someone else has allocated it
         * already.  The size better be the same as the size we are trying to
         * initialize to, or there is a name conflict (or worse).
         */
        if (result->size != size)
        {
            LWLockRelease(ShmemIndexLock);
            ereport(ERROR,
                    (errmsg("ShmemIndex entry size is wrong for data structure"
                            " \"%s\": expected %zu, actual %zu",
                            name, size, result->size)));
        }
        structPtr = result->location;
    }
    else
    {
        /* It isn't in the table yet. allocate and initialize it */
        structPtr = ShmemAllocNoError(size);
        if (structPtr == NULL)
        {
            /* out of memory; remove the failed ShmemIndex entry */
            hash_search(ShmemIndex, name, HASH_REMOVE, NULL);
            LWLockRelease(ShmemIndexLock);
            ereport(ERROR,
                    (errcode(ERRCODE_OUT_OF_MEMORY),
                     errmsg("not enough shared memory for data structure"
                            " \"%s\" (%zu bytes requested)",
                            name, size)));
        }
        result->size = size;
        result->location = structPtr;
    }

    LWLockRelease(ShmemIndexLock);

    Assert(ShmemAddrIsValid(structPtr));

    Assert(structPtr == (void *) CACHELINEALIGN(structPtr));

    return structPtr;
}


/*
 * Add two Size values, checking for overflow
 */
Size
add_size(Size s1, Size s2)
{
    Size        result;

    result = s1 + s2;
    /* We are assuming Size is an unsigned type here... */
    if (result < s1 || result < s2)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("requested shared memory size overflows size_t")));
    return result;
}

/*
 * Multiply two Size values, checking for overflow
 */
Size
mul_size(Size s1, Size s2)
{
    Size        result;

    if (s1 == 0 || s2 == 0)
        return 0;
    result = s1 * s2;
    /* We are assuming Size is an unsigned type here... */
    if (result / s2 != s1)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("requested shared memory size overflows size_t")));
    return result;
}