/*
* PGSharedMemoryCreate
*
* Create a shared memory segment of the given size and initialize its
* standard header. Also, register an on_shmem_exit callback to release
* the storage.
*
* Dead Postgres segments are recycled if found, but we do not fail upon
* collision with non-Postgres shmem segments. The idea here is to detect and
* re-use keys that may have been assigned by a crashed postmaster or backend.
*
* makePrivate means to always create a new segment, rather than attach to
* or recycle any existing segment.
*
* The port number is passed for possible use as a key (for SysV, we use
* it to generate the starting shmem key). In a standalone backend,
* zero will be passed.
*/
PGShmemHeader *
PGSharedMemoryCreate(Size size, bool makePrivate, int port)
{
IpcMemoryKey NextShmemSegID;
void *memAddress;
PGShmemHeader *hdr;
IpcMemoryId shmid;
struct stat statbuf;
Size sysvsize;
#if defined(EXEC_BACKEND) || !defined(MAP_HUGETLB)
if (huge_pages == HUGE_PAGES_ON)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("huge pages not supported on this platform")));
#endif
/* Room for a header? */
Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
/*
* As of PostgreSQL 9.3, we normally allocate only a very small amount of
* System V shared memory, and only for the purposes of providing an
* interlock to protect the data directory. The real shared memory block
* is allocated using mmap(). This works around the problem that many
* systems have very low limits on the amount of System V shared memory
* that can be allocated. Even a limit of a few megabytes will be enough
* to run many copies of PostgreSQL without needing to adjust system
* settings.
*
* We assume that no one will attempt to run PostgreSQL 9.3 or later on
* systems that are ancient enough that anonymous shared memory is not
* supported, such as pre-2.4 versions of Linux. If that turns out to be
* false, we might need to add a run-time test here and do this only if
* the running kernel supports it.
*
* However, we disable this logic in the EXEC_BACKEND case, and fall back
* to the old method of allocating the entire segment using System V
* shared memory, because there's no way to attach an mmap'd segment to a
* process after exec(). Since EXEC_BACKEND is intended only for
* developer use, this shouldn't be a big problem.
*/
#ifndef EXEC_BACKEND
AnonymousShmem = CreateAnonymousSegment(&size);
AnonymousShmemSize = size;
/* Now we need only allocate a minimal-sized SysV shmem block. */
sysvsize = sizeof(PGShmemHeader);
#else
sysvsize = size;
#endif
/* Make sure PGSharedMemoryAttach doesn't fail without need */
UsedShmemSegAddr = NULL;
/* Loop till we find a free IPC key */
NextShmemSegID = port * 1000;
for (NextShmemSegID++;; NextShmemSegID++)
{
/* Try to create new segment */
memAddress = InternalIpcMemoryCreate(NextShmemSegID, sysvsize);
if (memAddress)
break; /* successful create and attach */
/* Check shared memory and possibly remove and recreate */
if (makePrivate) /* a standalone backend shouldn't do this */
continue;
if ((memAddress = PGSharedMemoryAttach(NextShmemSegID, &shmid)) == NULL)
continue; /* can't attach, not one of mine */
/*
* If I am not the creator and it belongs to an extant process,
* continue.
*/
hdr = (PGShmemHeader *) memAddress;
if (hdr->creatorPID != getpid())
{
if (kill(hdr->creatorPID, 0) == 0 || errno != ESRCH)
{
shmdt(memAddress);
continue; /* segment belongs to a live process */
}
}
/*
* The segment appears to be from a dead Postgres process, or from a
* previous cycle of life in this same process. Zap it, if possible.
* This probably shouldn't fail, but if it does, assume the segment
* belongs to someone else after all, and continue quietly.
*/
shmdt(memAddress);
if (shmctl(shmid, IPC_RMID, NULL) < 0)
continue;
/*
* Now try again to create the segment.
*/
memAddress = InternalIpcMemoryCreate(NextShmemSegID, sysvsize);
if (memAddress)
break; /* successful create and attach */
/*
* Can only get here if some other process managed to create the same
* shmem key before we did. Let him have that one, loop around to try
* next key.
*/
}
/*
* 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;
/* Fill in the data directory ID info, too */
if (stat(DataDir, &statbuf) < 0)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not stat data directory \"%s\": %m",
DataDir)));
hdr->device = statbuf.st_dev;
hdr->inode = statbuf.st_ino;
/*
* Initialize space allocation status for segment.
*/
hdr->totalsize = size;
hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
/* Save info for possible future use */
UsedShmemSegAddr = memAddress;
UsedShmemSegID = (unsigned long) NextShmemSegID;
/*
* If AnonymousShmem is NULL here, then we're not using anonymous shared
* memory, and should return a pointer to the System V shared memory
* block. Otherwise, the System V shared memory block is only a shim, and
* we must return a pointer to the real block.
*/
if (AnonymousShmem == NULL)
return hdr;
memcpy(AnonymousShmem, hdr, sizeof(PGShmemHeader));
return (PGShmemHeader *) AnonymousShmem;
}
/*
* PGSharedMemoryCreate
*
* Create a shared memory segment of the given size and initialize its
* standard header. Also, register an on_shmem_exit callback to release
* the storage.
*
* Dead Postgres segments are recycled if found, but we do not fail upon
* collision with non-Postgres shmem segments. The idea here is to detect and
* re-use keys that may have been assigned by a crashed postmaster or backend.
*
* makePrivate means to always create a new segment, rather than attach to
* or recycle any existing segment.
*
* The port number is passed for possible use as a key (for SysV, we use
* it to generate the starting shmem key). In a standalone backend,
* zero will be passed.
*/
PGShmemHeader *
PGSharedMemoryCreate(Size size, bool makePrivate, int port,
PGShmemHeader **shim)
{
IpcMemoryKey NextShmemSegID;
void *memAddress;
PGShmemHeader *hdr;
IpcMemoryId shmid;
struct stat statbuf;
Size sysvsize;
/* Complain if hugepages demanded but we can't possibly support them */
#if !defined(USE_ANONYMOUS_SHMEM) || !defined(MAP_HUGETLB)
if (huge_pages == HUGE_PAGES_ON)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("huge pages not supported on this platform")));
#endif
/* Room for a header? */
Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
#ifdef USE_ANONYMOUS_SHMEM
AnonymousShmem = CreateAnonymousSegment(&size);
AnonymousShmemSize = size;
/* Register on-exit routine to unmap the anonymous segment */
on_shmem_exit(AnonymousShmemDetach, (Datum) 0);
/* Now we need only allocate a minimal-sized SysV shmem block. */
sysvsize = sizeof(PGShmemHeader);
#else
sysvsize = size;
#endif
/* Make sure PGSharedMemoryAttach doesn't fail without need */
UsedShmemSegAddr = NULL;
/* Loop till we find a free IPC key */
NextShmemSegID = port * 1000;
for (NextShmemSegID++;; NextShmemSegID++)
{
/* Try to create new segment */
memAddress = InternalIpcMemoryCreate(NextShmemSegID, sysvsize);
if (memAddress)
break; /* successful create and attach */
/* Check shared memory and possibly remove and recreate */
if (makePrivate) /* a standalone backend shouldn't do this */
continue;
if ((memAddress = PGSharedMemoryAttach(NextShmemSegID, &shmid)) == NULL)
continue; /* can't attach, not one of mine */
/*
* If I am not the creator and it belongs to an extant process,
* continue.
*/
hdr = (PGShmemHeader *) memAddress;
if (hdr->creatorPID != getpid())
{
if (kill(hdr->creatorPID, 0) == 0 || errno != ESRCH)
{
shmdt(memAddress);
continue; /* segment belongs to a live process */
}
}
/*
* The segment appears to be from a dead Postgres process, or from a
* previous cycle of life in this same process. Zap it, if possible,
* and any associated dynamic shared memory segments, as well. This
* probably shouldn't fail, but if it does, assume the segment belongs
* to someone else after all, and continue quietly.
*/
if (hdr->dsm_control != 0)
dsm_cleanup_using_control_segment(hdr->dsm_control);
shmdt(memAddress);
if (shmctl(shmid, IPC_RMID, NULL) < 0)
continue;
/*
* Now try again to create the segment.
*/
memAddress = InternalIpcMemoryCreate(NextShmemSegID, sysvsize);
if (memAddress)
break; /* successful create and attach */
/*
* Can only get here if some other process managed to create the same
* shmem key before we did. Let him have that one, loop around to try
* next key.
*/
}
/*
* 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;
hdr->dsm_control = 0;
/* Fill in the data directory ID info, too */
if (stat(DataDir, &statbuf) < 0)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not stat data directory \"%s\": %m",
DataDir)));
hdr->device = statbuf.st_dev;
hdr->inode = statbuf.st_ino;
/*
* Initialize space allocation status for segment.
*/
hdr->totalsize = size;
hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
*shim = hdr;
/* Save info for possible future use */
UsedShmemSegAddr = memAddress;
UsedShmemSegID = (unsigned long) NextShmemSegID;
/*
* If AnonymousShmem is NULL here, then we're not using anonymous shared
* memory, and should return a pointer to the System V shared memory
* block. Otherwise, the System V shared memory block is only a shim, and
* we must return a pointer to the real block.
*/
#ifdef USE_ANONYMOUS_SHMEM
if (AnonymousShmem == NULL)
return hdr;
memcpy(AnonymousShmem, hdr, sizeof(PGShmemHeader));
return (PGShmemHeader *) AnonymousShmem;
#else
return hdr;
#endif
}
