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Move shmem allocator's fields from PGShmemHeader to its own struct

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For readability. It was a slight modularity violation to have fields
in PGShmemHeader that were only used by the allocator code in
shmem.c. And it was inconsistent that ShmemLock was nevertheless not
stored there. Moving all the allocator-related fields to a separate
struct makes it more consistent and modular, and removes the need to
allocate and pass ShmemLock separately via BackendParameters.

Merge InitShmemAccess() and InitShmemAllocation() into a single
function that initializes the struct when called from postmaster, and
when called from backends in EXEC_BACKEND mode, re-establishes the
global variables. That's similar to all the *ShmemInit() functions
that we have.

Co-authored-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAExHW5uNRB9oT4pdo54qAo025MXFX4MfYrD9K15OCqe-ExnNvg@mail.gmail.com
This commit is contained in:
Heikki Linnakangas
2026-01-30 18:22:56 +02:00
parent e76221bd95
commit e2362eb2bd
8 changed files with 83 additions and 103 deletions
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2
src/backend/port/sysv_shmem.c
View File

@ -855,7 +855,7 @@ PGSharedMemoryCreate(Size size,
* Initialize space allocation status for segment.
*/
hdr->totalsize = size;
hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
hdr->content_offset = MAXALIGN(sizeof(PGShmemHeader));
*shim = hdr;
/* Save info for possible future use */

2
src/backend/port/win32_shmem.c
View File

@ -389,7 +389,7 @@ retry:
* Initialize space allocation status for segment.
*/
hdr->totalsize = size;
hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
hdr->content_offset = MAXALIGN(sizeof(PGShmemHeader));
hdr->dsm_control = 0;
/* Save info for possible future use */

7
src/backend/postmaster/launch_backend.c
View File

@ -96,7 +96,6 @@ typedef struct
HANDLE UsedShmemSegID;
#endif
void *UsedShmemSegAddr;
slock_t *ShmemLock;
#ifdef USE_INJECTION_POINTS
struct InjectionPointsCtl *ActiveInjectionPoints;
#endif
@ -676,7 +675,7 @@ SubPostmasterMain(int argc, char *argv[])
/* Restore basic shared memory pointers */
if (UsedShmemSegAddr != NULL)
InitShmemAccess(UsedShmemSegAddr);
InitShmemAllocator(UsedShmemSegAddr);
/*
* Run the appropriate Main function
@ -724,8 +723,6 @@ save_backend_variables(BackendParameters *param,
param->UsedShmemSegID = UsedShmemSegID;
param->UsedShmemSegAddr = UsedShmemSegAddr;
param->ShmemLock = ShmemLock;
#ifdef USE_INJECTION_POINTS
param->ActiveInjectionPoints = ActiveInjectionPoints;
#endif
@ -986,8 +983,6 @@ restore_backend_variables(BackendParameters *param)
UsedShmemSegID = param->UsedShmemSegID;
UsedShmemSegAddr = param->UsedShmemSegAddr;
ShmemLock = param->ShmemLock;
#ifdef USE_INJECTION_POINTS
ActiveInjectionPoints = param->ActiveInjectionPoints;
#endif

4
src/backend/storage/ipc/ipci.c
View File

@ -212,12 +212,10 @@ CreateSharedMemoryAndSemaphores(void)
Assert(strcmp("unknown",
GetConfigOption("huge_pages_status", false, false)) != 0);
InitShmemAccess(seghdr);
/*
* Set up shared memory allocation mechanism
*/
InitShmemAllocation();
InitShmemAllocator(seghdr);
/* Initialize subsystems */
CreateOrAttachShmemStructs();

163
src/backend/storage/ipc/shmem.c
View File

@ -76,20 +76,33 @@
#include "storage/spin.h"
#include "utils/builtins.h"
/*
* 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 */
HTAB *index; /* copy of ShmemIndex */
/* protects shared memory and LWLock allocation */
slock_t shmem_lock;
} ShmemAllocatorData;
static void *ShmemAllocRaw(Size size, Size *allocated_size);
static void *ShmemAllocUnlocked(Size 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 */
slock_t *ShmemLock; /* spinlock for shared memory and LWLock
* allocation */
static ShmemAllocatorData *ShmemAllocator;
slock_t *ShmemLock; /* points to ShmemAllocator->shmem_lock */
static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */
/* To get reliable results for NUMA inquiry we need to "touch pages" once */
@ -98,49 +111,64 @@ static bool firstNumaTouch = true;
Datum pg_numa_available(PG_FUNCTION_ARGS);
/*
* InitShmemAccess() --- set up basic pointers to shared memory.
* 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 shared
* memory areas.
*/
void
InitShmemAccess(PGShmemHeader *seghdr)
InitShmemAllocator(PGShmemHeader *seghdr)
{
Assert(seghdr != NULL);
/*
* 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));
ShmemSegHdr = seghdr;
ShmemBase = seghdr;
ShmemEnd = (char *) ShmemBase + seghdr->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;
#ifndef EXEC_BACKEND
Assert(!IsUnderPostmaster);
#endif
if (IsUnderPostmaster)
{
PGShmemHeader *shmhdr = ShmemSegHdr;
Assert(shmhdr != NULL);
ShmemAllocator = (ShmemAllocatorData *) ((char *) shmhdr + shmhdr->content_offset);
ShmemLock = &ShmemAllocator->shmem_lock;
}
else
{
Size offset;
/*
* 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));
/*
* 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)));
SpinLockInit(ShmemLock);
ShmemAllocator = (ShmemAllocatorData *) ((char *) seghdr + 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.
*/
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;
SpinLockInit(&ShmemAllocator->shmem_lock);
ShmemLock = &ShmemAllocator->shmem_lock;
ShmemAllocator->free_offset = offset;
/* ShmemIndex can't be set up yet (need LWLocks first) */
ShmemAllocator->index = NULL;
ShmemIndex = (HTAB *) NULL;
}
}
/*
@ -209,13 +237,13 @@ ShmemAllocRaw(Size size, Size *allocated_size)
SpinLockAcquire(ShmemLock);
newStart = ShmemSegHdr->freeoffset;
newStart = ShmemAllocator->free_offset;
newFree = newStart + size;
if (newFree <= ShmemSegHdr->totalsize)
{
newSpace = (char *) ShmemBase + newStart;
ShmemSegHdr->freeoffset = newFree;
ShmemAllocator->free_offset = newFree;
}
else
newSpace = NULL;
@ -228,45 +256,6 @@ ShmemAllocRaw(Size size, Size *allocated_size)
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.
*/
static 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 = (char *) ShmemBase + newStart;
Assert(newSpace == (void *) MAXALIGN(newSpace));
return newSpace;
}
/*
* ShmemAddrIsValid -- test if an address refers to shared memory
*
@ -395,16 +384,14 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
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;
Assert(ShmemAllocator->index != NULL);
structPtr = ShmemAllocator->index;
*foundPtr = true;
}
else
@ -417,9 +404,9 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
* index has been initialized. This should be OK because no other
* process can be accessing shared memory yet.
*/
Assert(shmemseghdr->index == NULL);
Assert(ShmemAllocator->index == NULL);
structPtr = ShmemAlloc(size);
shmemseghdr->index = structPtr;
ShmemAllocator->index = structPtr;
*foundPtr = false;
}
LWLockRelease(ShmemIndexLock);
@ -553,15 +540,15 @@ pg_get_shmem_allocations(PG_FUNCTION_ARGS)
/* output shared memory allocated but not counted via the shmem index */
values[0] = CStringGetTextDatum("<anonymous>");
nulls[1] = true;
values[2] = Int64GetDatum(ShmemSegHdr->freeoffset - named_allocated);
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(ShmemSegHdr->freeoffset);
values[1] = Int64GetDatum(ShmemAllocator->free_offset);
nulls[1] = false;
values[2] = Int64GetDatum(ShmemSegHdr->totalsize - ShmemSegHdr->freeoffset);
values[2] = Int64GetDatum(ShmemSegHdr->totalsize - ShmemAllocator->free_offset);
values[3] = values[2];
tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);

4
src/include/storage/pg_shmem.h
View File

@ -32,9 +32,9 @@ typedef struct PGShmemHeader /* standard header for all Postgres shmem */
#define PGShmemMagic 679834894
pid_t creatorPID; /* PID of creating process (set but unread) */
Size totalsize; /* total size of segment */
Size freeoffset; /* offset to first free space */
Size content_offset; /* offset to the data, i.e. size of this
* header */
dsm_handle dsm_control; /* ID of dynamic shared memory control seg */
void *index; /* pointer to ShmemIndex table */
#ifndef WIN32 /* Windows doesn't have useful inode#s */
dev_t device; /* device data directory is on */
ino_t inode; /* inode number of data directory */

3
src/include/storage/shmem.h
View File

@ -29,8 +29,7 @@
extern PGDLLIMPORT slock_t *ShmemLock;
typedef struct PGShmemHeader PGShmemHeader; /* avoid including
* storage/pg_shmem.h here */
extern void InitShmemAccess(PGShmemHeader *seghdr);
extern void InitShmemAllocation(void);
extern void InitShmemAllocator(PGShmemHeader *seghdr);
extern void *ShmemAlloc(Size size);
extern void *ShmemAllocNoError(Size size);
extern bool ShmemAddrIsValid(const void *addr);

1
src/tools/pgindent/typedefs.list
View File

@ -2804,6 +2804,7 @@ SharedTypmodTableEntry
Sharedsort
ShellTypeInfo
ShippableCacheEntry
ShmemAllocatorData
ShippableCacheKey
ShmemIndexEnt
ShutdownForeignScan_function