From e2362eb2bd1459319dacaeaa5dc886dbca546b96 Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Fri, 30 Jan 2026 18:22:56 +0200 Subject: [PATCH] Move shmem allocator's fields from PGShmemHeader to its own struct 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 Discussion: https://www.postgresql.org/message-id/CAExHW5uNRB9oT4pdo54qAo025MXFX4MfYrD9K15OCqe-ExnNvg@mail.gmail.com --- src/backend/port/sysv_shmem.c | 2 +- src/backend/port/win32_shmem.c | 2 +- src/backend/postmaster/launch_backend.c | 7 +- src/backend/storage/ipc/ipci.c | 4 +- src/backend/storage/ipc/shmem.c | 163 +++++++++++------------- src/include/storage/pg_shmem.h | 4 +- src/include/storage/shmem.h | 3 +- src/tools/pgindent/typedefs.list | 1 + 8 files changed, 83 insertions(+), 103 deletions(-) diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c index 3cd3544fa2b..2e3886cf9fe 100644 --- a/src/backend/port/sysv_shmem.c +++ b/src/backend/port/sysv_shmem.c @@ -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 */ diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c index 7cb8b4c9b60..794e4fcb2ad 100644 --- a/src/backend/port/win32_shmem.c +++ b/src/backend/port/win32_shmem.c @@ -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 */ diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c index cea229ad6a4..45690b11c99 100644 --- a/src/backend/postmaster/launch_backend.c +++ b/src/backend/postmaster/launch_backend.c @@ -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 diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c index 2a3dfedf7e9..1f7e933d500 100644 --- a/src/backend/storage/ipc/ipci.c +++ b/src/backend/storage/ipc/ipci.c @@ -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(); diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c index 1b536363152..9f362ce8641 100644 --- a/src/backend/storage/ipc/shmem.c +++ b/src/backend/storage/ipc/shmem.c @@ -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(""); 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); diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h index 3aeada554b2..10c7b065861 100644 --- a/src/include/storage/pg_shmem.h +++ b/src/include/storage/pg_shmem.h @@ -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 */ diff --git a/src/include/storage/shmem.h b/src/include/storage/shmem.h index e71a51dfe84..89d45287c17 100644 --- a/src/include/storage/shmem.h +++ b/src/include/storage/shmem.h @@ -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); diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list index 34374df0d67..9f5ee8fd482 100644 --- a/src/tools/pgindent/typedefs.list +++ b/src/tools/pgindent/typedefs.list @@ -2804,6 +2804,7 @@ SharedTypmodTableEntry Sharedsort ShellTypeInfo ShippableCacheEntry +ShmemAllocatorData ShippableCacheKey ShmemIndexEnt ShutdownForeignScan_function -- 2.47.3