win32_shmem.c

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/*-------------------------------------------------------------------------
 *
 * win32_shmem.c
 *    Implement shared memory using win32 facilities
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/port/win32_shmem.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "miscadmin.h"
#include "storage/dsm.h"
#include "storage/ipc.h"
#include "storage/pg_shmem.h"
#include "utils/guc_hooks.h"
 
 
/*
 * Early in a process's life, Windows asynchronously creates threads for the
 * process's "default thread pool"
 * (https://docs.microsoft.com/en-us/windows/desktop/ProcThread/thread-pools).
 * Occasionally, thread creation allocates a stack after
 * PGSharedMemoryReAttach() has released UsedShmemSegAddr and before it has
 * mapped shared memory at UsedShmemSegAddr.  This would cause mapping to fail
 * if the allocator preferred the just-released region for allocating the new
 * thread stack.  We observed such failures in some Windows Server 2016
 * configurations.  To give the system another region to prefer, reserve and
 * release an additional, protective region immediately before reserving or
 * releasing shared memory.  The idea is that, if the allocator handed out
 * REGION1 pages before REGION2 pages at one occasion, it will do so whenever
 * both regions are free.  Windows Server 2016 exhibits that behavior, and a
 * system behaving differently would have less need to protect
 * UsedShmemSegAddr.  The protective region must be at least large enough for
 * one thread stack.  However, ten times as much is less than 2% of the 32-bit
 * address space and is negligible relative to the 64-bit address space.
 */
#define PROTECTIVE_REGION_SIZE (10 * WIN32_STACK_RLIMIT)
void       *ShmemProtectiveRegion = NULL;
 
HANDLE      UsedShmemSegID = INVALID_HANDLE_VALUE;
void       *UsedShmemSegAddr = NULL;
static Size UsedShmemSegSize = 0;
 
static bool EnableLockPagesPrivilege(int elevel);
static void pgwin32_SharedMemoryDelete(int status, Datum shmId);
 
/*
 * Generate shared memory segment name. Expand the data directory, to generate
 * an identifier unique for this data directory. Then replace all backslashes
 * with forward slashes, since backslashes aren't permitted in global object names.
 *
 * Store the shared memory segment in the Global\ namespace (requires NT2 TSE or
 * 2000, but that's all we support for other reasons as well), to make sure you can't
 * open two postmasters in different sessions against the same data directory.
 *
 * XXX: What happens with junctions? It's only someone breaking things on purpose,
 *      and this is still better than before, but we might want to do something about
 *      that sometime in the future.
 */
static char *
GetSharedMemName(void)
{
    char       *retptr;
    DWORD       bufsize;
    DWORD       r;
    char       *cp;
 
    bufsize = GetFullPathName(DataDir, 0, NULL, NULL);
    if (bufsize == 0)
        elog(FATAL, "could not get size for full pathname of datadir %s: error code %lu",
             DataDir, GetLastError());
 
    retptr = malloc(bufsize + 18);  /* 18 for Global\PostgreSQL: */
    if (retptr == NULL)
        elog(FATAL, "could not allocate memory for shared memory name");
 
    strcpy(retptr, "Global\\PostgreSQL:");
    r = GetFullPathName(DataDir, bufsize, retptr + 18, NULL);
    if (r == 0 || r > bufsize)
        elog(FATAL, "could not generate full pathname for datadir %s: error code %lu",
             DataDir, GetLastError());
 
    /*
     * XXX: Intentionally overwriting the Global\ part here. This was not the
     * original approach, but putting it in the actual Global\ namespace
     * causes permission errors in a lot of cases, so we leave it in the
     * default namespace for now.
     */
    for (cp = retptr; *cp; cp++)
        if (*cp == '\\')
            *cp = '/';
 
    return retptr;
}
 
 
/*
 * PGSharedMemoryIsInUse
 *
 * Is a previously-existing shmem segment still existing and in use?
 *
 * The point of this exercise is to detect the case where a prior postmaster
 * crashed, but it left child backends that are still running.  Therefore
 * we only care about shmem segments that are associated with the intended
 * DataDir.  This is an important consideration since accidental matches of
 * shmem segment IDs are reasonably common.
 */
bool
PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
{
    char       *szShareMem;
    HANDLE      hmap;
 
    szShareMem = GetSharedMemName();
 
    hmap = OpenFileMapping(FILE_MAP_READ, FALSE, szShareMem);
 
    free(szShareMem);
 
    if (hmap == NULL)
        return false;
 
    CloseHandle(hmap);
    return true;
}
 
/*
 * EnableLockPagesPrivilege
 *
 * Try to acquire SeLockMemoryPrivilege so we can use large pages.
 */
static bool
EnableLockPagesPrivilege(int elevel)
{
    HANDLE      hToken;
    TOKEN_PRIVILEGES tp;
    LUID        luid;
 
    if (!OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken))
    {
        ereport(elevel,
                (errmsg("could not enable user right \"%s\": error code %lu",
 
        /*
         * translator: This is a term from Windows and should be translated to
         * match the Windows localization.
         */
                        _("Lock pages in memory"),
                        GetLastError()),
                 errdetail("Failed system call was %s.", "OpenProcessToken")));
        return FALSE;
    }
 
    if (!LookupPrivilegeValue(NULL, SE_LOCK_MEMORY_NAME, &luid))
    {
        ereport(elevel,
                (errmsg("could not enable user right \"%s\": error code %lu", _("Lock pages in memory"), GetLastError()),
                 errdetail("Failed system call was %s.", "LookupPrivilegeValue")));
        CloseHandle(hToken);
        return FALSE;
    }
    tp.PrivilegeCount = 1;
    tp.Privileges[0].Luid = luid;
    tp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
 
    if (!AdjustTokenPrivileges(hToken, FALSE, &tp, 0, NULL, NULL))
    {
        ereport(elevel,
                (errmsg("could not enable user right \"%s\": error code %lu", _("Lock pages in memory"), GetLastError()),
                 errdetail("Failed system call was %s.", "AdjustTokenPrivileges")));
        CloseHandle(hToken);
        return FALSE;
    }
 
    if (GetLastError() != ERROR_SUCCESS)
    {
        if (GetLastError() == ERROR_NOT_ALL_ASSIGNED)
            ereport(elevel,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("could not enable user right \"%s\"", _("Lock pages in memory")),
                     errhint("Assign user right \"%s\" to the Windows user account which runs PostgreSQL.",
                             _("Lock pages in memory"))));
        else
            ereport(elevel,
                    (errmsg("could not enable user right \"%s\": error code %lu", _("Lock pages in memory"), GetLastError()),
                     errdetail("Failed system call was %s.", "AdjustTokenPrivileges")));
        CloseHandle(hToken);
        return FALSE;
    }
 
    CloseHandle(hToken);
 
    return TRUE;
}
 
/*
 * PGSharedMemoryCreate
 *
 * Create a shared memory segment of the given size and initialize its
 * standard header.
 */
PGShmemHeader *
PGSharedMemoryCreate(Size size,
                     PGShmemHeader **shim)
{
    void       *memAddress;
    PGShmemHeader *hdr;
    HANDLE      hmap,
                hmap2;
    char       *szShareMem;
    int         i;
    DWORD       size_high;
    DWORD       size_low;
    SIZE_T      largePageSize = 0;
    Size        orig_size = size;
    DWORD       flProtect = PAGE_READWRITE;
    DWORD       desiredAccess;
 
    ShmemProtectiveRegion = VirtualAlloc(NULL, PROTECTIVE_REGION_SIZE,
                                         MEM_RESERVE, PAGE_NOACCESS);
    if (ShmemProtectiveRegion == NULL)
        elog(FATAL, "could not reserve memory region: error code %lu",
             GetLastError());
 
    /* Room for a header? */
    Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
 
    szShareMem = GetSharedMemName();
 
    UsedShmemSegAddr = NULL;
 
    if (huge_pages == HUGE_PAGES_ON || huge_pages == HUGE_PAGES_TRY)
    {
        /* Does the processor support large pages? */
        largePageSize = GetLargePageMinimum();
        if (largePageSize == 0)
        {
            ereport(huge_pages == HUGE_PAGES_ON ? FATAL : DEBUG1,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("the processor does not support large pages")));
            ereport(DEBUG1,
                    (errmsg_internal("disabling huge pages")));
        }
        else if (!EnableLockPagesPrivilege(huge_pages == HUGE_PAGES_ON ? FATAL : DEBUG1))
        {
            ereport(DEBUG1,
                    (errmsg_internal("disabling huge pages")));
        }
        else
        {
            /* Huge pages available and privilege enabled, so turn on */
            flProtect = PAGE_READWRITE | SEC_COMMIT | SEC_LARGE_PAGES;
 
            /* Round size up as appropriate. */
            if (size % largePageSize != 0)
                size += largePageSize - (size % largePageSize);
        }
    }
 
retry:
#ifdef _WIN64
    size_high = size >> 32;
#else
    size_high = 0;
#endif
    size_low = (DWORD) size;
 
    /*
     * When recycling a shared memory segment, it may take a short while
     * before it gets dropped from the global namespace. So re-try after
     * sleeping for a second, and continue retrying 10 times. (both the 1
     * second time and the 10 retries are completely arbitrary)
     */
    for (i = 0; i < 10; i++)
    {
        /*
         * In case CreateFileMapping() doesn't set the error code to 0 on
         * success
         */
        SetLastError(0);
 
        hmap = CreateFileMapping(INVALID_HANDLE_VALUE,  /* Use the pagefile */
                                 NULL,  /* Default security attrs */
                                 flProtect,
                                 size_high, /* Size Upper 32 Bits   */
                                 size_low,  /* Size Lower 32 bits */
                                 szShareMem);
 
        if (!hmap)
        {
            if (GetLastError() == ERROR_NO_SYSTEM_RESOURCES &&
                huge_pages == HUGE_PAGES_TRY &&
                (flProtect & SEC_LARGE_PAGES) != 0)
            {
                elog(DEBUG1, "CreateFileMapping(%zu) with SEC_LARGE_PAGES failed, "
                     "huge pages disabled",
                     size);
 
                /*
                 * Use the original size, not the rounded-up value, when
                 * falling back to non-huge pages.
                 */
                size = orig_size;
                flProtect = PAGE_READWRITE;
                goto retry;
            }
            else
                ereport(FATAL,
                        (errmsg("could not create shared memory segment: error code %lu", GetLastError()),
                         errdetail("Failed system call was CreateFileMapping(size=%zu, name=%s).",
                                   size, szShareMem)));
        }
 
        /*
         * If the segment already existed, CreateFileMapping() will return a
         * handle to the existing one and set ERROR_ALREADY_EXISTS.
         */
        if (GetLastError() == ERROR_ALREADY_EXISTS)
        {
            CloseHandle(hmap);  /* Close the handle, since we got a valid one
                                 * to the previous segment. */
            hmap = NULL;
            Sleep(1000);
            continue;
        }
        break;
    }
 
    /*
     * If the last call in the loop still returned ERROR_ALREADY_EXISTS, this
     * shared memory segment exists and we assume it belongs to somebody else.
     */
    if (!hmap)
        ereport(FATAL,
                (errmsg("pre-existing shared memory block is still in use"),
                 errhint("Check if there are any old server processes still running, and terminate them.")));
 
    free(szShareMem);
 
    /*
     * Make the handle inheritable
     */
    if (!DuplicateHandle(GetCurrentProcess(), hmap, GetCurrentProcess(), &hmap2, 0, TRUE, DUPLICATE_SAME_ACCESS))
        ereport(FATAL,
                (errmsg("could not create shared memory segment: error code %lu", GetLastError()),
                 errdetail("Failed system call was DuplicateHandle.")));
 
    /*
     * Close the old, non-inheritable handle. If this fails we don't really
     * care.
     */
    if (!CloseHandle(hmap))
        elog(LOG, "could not close handle to shared memory: error code %lu", GetLastError());
 
    desiredAccess = FILE_MAP_WRITE | FILE_MAP_READ;
 
#ifdef FILE_MAP_LARGE_PAGES
    /* Set large pages if wanted. */
    if ((flProtect & SEC_LARGE_PAGES) != 0)
        desiredAccess |= FILE_MAP_LARGE_PAGES;
#endif
 
    /*
     * Get a pointer to the new shared memory segment. Map the whole segment
     * at once, and let the system decide on the initial address.
     */
    memAddress = MapViewOfFileEx(hmap2, desiredAccess, 0, 0, 0, NULL);
    if (!memAddress)
        ereport(FATAL,
                (errmsg("could not create shared memory segment: error code %lu", GetLastError()),
                 errdetail("Failed system call was MapViewOfFileEx.")));
 
 
 
    /*
     * OK, we created a new segment.  Mark it as created by this process. The
     * order of assignments here is critical so that another Postgres process
     * can't see the header as valid but belonging to an invalid PID!
     */
    hdr = (PGShmemHeader *) memAddress;
    hdr->creatorPID = getpid();
    hdr->magic = PGShmemMagic;
 
    /*
     * Initialize space allocation status for segment.
     */
    hdr->totalsize = size;
    hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
    hdr->dsm_control = 0;
 
    /* Save info for possible future use */
    UsedShmemSegAddr = memAddress;
    UsedShmemSegSize = size;
    UsedShmemSegID = hmap2;
 
    /* Register on-exit routine to delete the new segment */
    on_shmem_exit(pgwin32_SharedMemoryDelete, PointerGetDatum(hmap2));
 
    *shim = hdr;
 
    /* Report whether huge pages are in use */
    SetConfigOption("huge_pages_status", (flProtect & SEC_LARGE_PAGES) ?
                    "on" : "off", PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 
    return hdr;
}
 
/*
 * PGSharedMemoryReAttach
 *
 * This is called during startup of a postmaster child process to re-attach to
 * an already existing shared memory segment, using the handle inherited from
 * the postmaster.
 *
 * ShmemProtectiveRegion, UsedShmemSegID and UsedShmemSegAddr are implicit
 * parameters to this routine.  The caller must have already restored them to
 * the postmaster's values.
 */
void
PGSharedMemoryReAttach(void)
{
    PGShmemHeader *hdr;
    void       *origUsedShmemSegAddr = UsedShmemSegAddr;
 
    Assert(ShmemProtectiveRegion != NULL);
    Assert(UsedShmemSegAddr != NULL);
    Assert(IsUnderPostmaster);
 
    /*
     * Release memory region reservations made by the postmaster
     */
    if (VirtualFree(ShmemProtectiveRegion, 0, MEM_RELEASE) == 0)
        elog(FATAL, "failed to release reserved memory region (addr=%p): error code %lu",
             ShmemProtectiveRegion, GetLastError());
    if (VirtualFree(UsedShmemSegAddr, 0, MEM_RELEASE) == 0)
        elog(FATAL, "failed to release reserved memory region (addr=%p): error code %lu",
             UsedShmemSegAddr, GetLastError());
 
    hdr = (PGShmemHeader *) MapViewOfFileEx(UsedShmemSegID, FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, 0, UsedShmemSegAddr);
    if (!hdr)
        elog(FATAL, "could not reattach to shared memory (key=%p, addr=%p): error code %lu",
             UsedShmemSegID, UsedShmemSegAddr, GetLastError());
    if (hdr != origUsedShmemSegAddr)
        elog(FATAL, "reattaching to shared memory returned unexpected address (got %p, expected %p)",
             hdr, origUsedShmemSegAddr);
    if (hdr->magic != PGShmemMagic)
        elog(FATAL, "reattaching to shared memory returned non-PostgreSQL memory");
    dsm_set_control_handle(hdr->dsm_control);
 
    UsedShmemSegAddr = hdr;     /* probably redundant */
}
 
/*
 * PGSharedMemoryNoReAttach
 *
 * This is called during startup of a postmaster child process when we choose
 * *not* to re-attach to the existing shared memory segment.  We must clean up
 * to leave things in the appropriate state.
 *
 * The child process startup logic might or might not call PGSharedMemoryDetach
 * after this; make sure that it will be a no-op if called.
 *
 * ShmemProtectiveRegion, UsedShmemSegID and UsedShmemSegAddr are implicit
 * parameters to this routine.  The caller must have already restored them to
 * the postmaster's values.
 */
void
PGSharedMemoryNoReAttach(void)
{
    Assert(ShmemProtectiveRegion != NULL);
    Assert(UsedShmemSegAddr != NULL);
    Assert(IsUnderPostmaster);
 
    /*
     * Under Windows we will not have mapped the segment, so we don't need to
     * un-map it.  Just reset UsedShmemSegAddr to show we're not attached.
     */
    UsedShmemSegAddr = NULL;
 
    /*
     * We *must* close the inherited shmem segment handle, else Windows will
     * consider the existence of this process to mean it can't release the
     * shmem segment yet.  We can now use PGSharedMemoryDetach to do that.
     */
    PGSharedMemoryDetach();
}
 
/*
 * PGSharedMemoryDetach
 *
 * Detach from the shared memory segment, if still attached.  This is not
 * intended to be called explicitly by the process that originally created the
 * segment (it will have an on_shmem_exit callback registered to do that).
 * Rather, this is for subprocesses that have inherited an attachment and want
 * to get rid of it.
 *
 * ShmemProtectiveRegion, UsedShmemSegID and UsedShmemSegAddr are implicit
 * parameters to this routine.
 */
void
PGSharedMemoryDetach(void)
{
    /*
     * Releasing the protective region liberates an unimportant quantity of
     * address space, but be tidy.
     */
    if (ShmemProtectiveRegion != NULL)
    {
        if (VirtualFree(ShmemProtectiveRegion, 0, MEM_RELEASE) == 0)
            elog(LOG, "failed to release reserved memory region (addr=%p): error code %lu",
                 ShmemProtectiveRegion, GetLastError());
 
        ShmemProtectiveRegion = NULL;
    }
 
    /* Unmap the view, if it's mapped */
    if (UsedShmemSegAddr != NULL)
    {
        if (!UnmapViewOfFile(UsedShmemSegAddr))
            elog(LOG, "could not unmap view of shared memory: error code %lu",
                 GetLastError());
 
        UsedShmemSegAddr = NULL;
    }
 
    /* And close the shmem handle, if we have one */
    if (UsedShmemSegID != INVALID_HANDLE_VALUE)
    {
        if (!CloseHandle(UsedShmemSegID))
            elog(LOG, "could not close handle to shared memory: error code %lu",
                 GetLastError());
 
        UsedShmemSegID = INVALID_HANDLE_VALUE;
    }
}
 
 
/*
 * pgwin32_SharedMemoryDelete
 *
 * Detach from and delete the shared memory segment
 * (called as an on_shmem_exit callback, hence funny argument list)
 */
static void
pgwin32_SharedMemoryDelete(int status, Datum shmId)
{
    Assert(DatumGetPointer(shmId) == UsedShmemSegID);
    PGSharedMemoryDetach();
}
 
/*
 * pgwin32_ReserveSharedMemoryRegion(hChild)
 *
 * Reserve the memory region that will be used for shared memory in a child
 * process. It is called before the child process starts, to make sure the
 * memory is available.
 *
 * Once the child starts, DLLs loading in different order or threads getting
 * scheduled differently may allocate memory which can conflict with the
 * address space we need for our shared memory. By reserving the shared
 * memory region before the child starts, and freeing it only just before we
 * attempt to get access to the shared memory forces these allocations to
 * be given different address ranges that don't conflict.
 *
 * NOTE! This function executes in the postmaster, and should for this
 * reason not use elog(FATAL) since that would take down the postmaster.
 */
int
pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
{
    void       *address;
 
    Assert(ShmemProtectiveRegion != NULL);
    Assert(UsedShmemSegAddr != NULL);
    Assert(UsedShmemSegSize != 0);
 
    /* ShmemProtectiveRegion */
    address = VirtualAllocEx(hChild, ShmemProtectiveRegion,
                             PROTECTIVE_REGION_SIZE,
                             MEM_RESERVE, PAGE_NOACCESS);
    if (address == NULL)
    {
        /* Don't use FATAL since we're running in the postmaster */
        elog(LOG, "could not reserve shared memory region (addr=%p) for child %p: error code %lu",
             ShmemProtectiveRegion, hChild, GetLastError());
        return false;
    }
    if (address != ShmemProtectiveRegion)
    {
        /*
         * Should never happen - in theory if allocation granularity causes
         * strange effects it could, so check just in case.
         *
         * Don't use FATAL since we're running in the postmaster.
         */
        elog(LOG, "reserved shared memory region got incorrect address %p, expected %p",
             address, ShmemProtectiveRegion);
        return false;
    }
 
    /* UsedShmemSegAddr */
    address = VirtualAllocEx(hChild, UsedShmemSegAddr, UsedShmemSegSize,
                             MEM_RESERVE, PAGE_READWRITE);
    if (address == NULL)
    {
        elog(LOG, "could not reserve shared memory region (addr=%p) for child %p: error code %lu",
             UsedShmemSegAddr, hChild, GetLastError());
        return false;
    }
    if (address != UsedShmemSegAddr)
    {
        elog(LOG, "reserved shared memory region got incorrect address %p, expected %p",
             address, UsedShmemSegAddr);
        return false;
    }
 
    return true;
}
 
/*
 * This function is provided for consistency with sysv_shmem.c and does not
 * provide any useful information for Windows.  To obtain the large page size,
 * use GetLargePageMinimum() instead.
 */
void
GetHugePageSize(Size *hugepagesize, int *mmap_flags)
{
    if (hugepagesize)
        *hugepagesize = 0;
    if (mmap_flags)
        *mmap_flags = 0;
}
 
/*
 * GUC check_hook for huge_page_size
 */
bool
check_huge_page_size(int *newval, void **extra, GucSource source)
{
    if (*newval != 0)
    {
        GUC_check_errdetail("\"huge_page_size\" must be 0 on this platform.");
        return false;
    }
    return true;
}