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Commit15fc5e1

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Clean up handling of anonymous mmap'd shared-memory segment.
Fix detaching of the mmap'd segment to have its own on_shmem_exit callback,rather than piggybacking on the one for detaching from the SysV segment.That was confusing, and given the distance between the two attach calls,it was trouble waiting to happen.Make the detaching calls idempotent by clearing AnonymousShmem to showwe've already unmapped. I spent quite a bit of time yesterday tryingto find a path that would allow the munmap()'s to be done twice, andwhile I did not succeed, it seems silly that there's even a question.Make the #ifdef logic less confusing by separating "do we want to useanonymous shmem" from EXEC_BACKEND. Even though there's no currentscenario where those conditions are different, it is not helpful fordifferent places in the same file to be testing EXEC_BACKEND for whatare fundamentally different reasons.Don't do on_exit_reset() in StartBackgroundWorker(). At best that'suseless (InitPostmasterChild would have done it already) and at worstit could zap some callback that's unrelated to shared memory.Improve comments, and simplify the huge_pages enablement logic slightly.Back-patch to 9.4 where hugepage support was introduced.Arguably this should go into 9.3 as well, but the code lookssignificantly different there, and I doubt it's worth thetrouble of adapting the patch given I can't show a live bug.
1 parent0a4bf6b commit15fc5e1

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2 files changed

+82
-46
lines changed

2 files changed

+82
-46
lines changed

‎src/backend/port/sysv_shmem.c

Lines changed: 82 additions & 45 deletions
Original file line numberDiff line numberDiff line change
@@ -3,8 +3,11 @@
33
* sysv_shmem.c
44
* Implement shared memory using SysV facilities
55
*
6-
* These routines represent a fairly thin layer on top of SysV shared
7-
* memory functionality.
6+
* These routines used to be a fairly thin layer on top of SysV shared
7+
* memory functionality. With the addition of anonymous-shmem logic,
8+
* they're a bit fatter now. We still require a SysV shmem block to
9+
* exist, though, because mmap'd shmem provides no way to find out how
10+
* many processes are attached, which we need for interlocking purposes.
811
*
912
* Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
1013
* Portions Copyright (c) 1994, Regents of the University of California
@@ -36,14 +39,44 @@
3639
#include"utils/guc.h"
3740

3841

42+
/*
43+
* As of PostgreSQL 9.3, we normally allocate only a very small amount of
44+
* System V shared memory, and only for the purposes of providing an
45+
* interlock to protect the data directory. The real shared memory block
46+
* is allocated using mmap(). This works around the problem that many
47+
* systems have very low limits on the amount of System V shared memory
48+
* that can be allocated. Even a limit of a few megabytes will be enough
49+
* to run many copies of PostgreSQL without needing to adjust system settings.
50+
*
51+
* We assume that no one will attempt to run PostgreSQL 9.3 or later on
52+
* systems that are ancient enough that anonymous shared memory is not
53+
* supported, such as pre-2.4 versions of Linux. If that turns out to be
54+
* false, we might need to add compile and/or run-time tests here and do this
55+
* only if the running kernel supports it.
56+
*
57+
* However, we must always disable this logic in the EXEC_BACKEND case, and
58+
* fall back to the old method of allocating the entire segment using System V
59+
* shared memory, because there's no way to attach an anonymous mmap'd segment
60+
* to a process after exec(). Since EXEC_BACKEND is intended only for
61+
* developer use, this shouldn't be a big problem. Because of this, we do
62+
* not worry about supporting anonymous shmem in the EXEC_BACKEND cases below.
63+
*/
64+
#ifndefEXEC_BACKEND
65+
#defineUSE_ANONYMOUS_SHMEM
66+
#endif
67+
68+
3969
typedefkey_tIpcMemoryKey;/* shared memory key passed to shmget(2) */
4070
typedefintIpcMemoryId;/* shared memory ID returned by shmget(2) */
4171

4272

4373
unsigned longUsedShmemSegID=0;
4474
void*UsedShmemSegAddr=NULL;
75+
76+
#ifdefUSE_ANONYMOUS_SHMEM
4577
staticSizeAnonymousShmemSize;
4678
staticvoid*AnonymousShmem=NULL;
79+
#endif
4780

4881
staticvoid*InternalIpcMemoryCreate(IpcMemoryKeymemKey,Sizesize);
4982
staticvoidIpcMemoryDetach(intstatus,Datumshmaddr);
@@ -204,10 +237,6 @@ IpcMemoryDetach(int status, Datum shmaddr)
204237
/* Detach System V shared memory block. */
205238
if (shmdt(DatumGetPointer(shmaddr))<0)
206239
elog(LOG,"shmdt(%p) failed: %m",DatumGetPointer(shmaddr));
207-
/* Release anonymous shared memory block, if any. */
208-
if (AnonymousShmem!=NULL
209-
&&munmap(AnonymousShmem,AnonymousShmemSize)<0)
210-
elog(LOG,"munmap(%p) failed: %m",AnonymousShmem);
211240
}
212241

213242
/****************************************************************************/
@@ -318,14 +347,15 @@ PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
318347
return true;
319348
}
320349

350+
#ifdefUSE_ANONYMOUS_SHMEM
351+
321352
/*
322353
* Creates an anonymous mmap()ed shared memory segment.
323354
*
324355
* Pass the requested size in *size. This function will modify *size to the
325356
* actual size of the allocation, if it ends up allocating a segment that is
326357
* larger than requested.
327358
*/
328-
#ifndefEXEC_BACKEND
329359
staticvoid*
330360
CreateAnonymousSegment(Size*size)
331361
{
@@ -334,10 +364,8 @@ CreateAnonymousSegment(Size *size)
334364
intmmap_errno=0;
335365

336366
#ifndefMAP_HUGETLB
337-
if (huge_pages==HUGE_PAGES_ON)
338-
ereport(ERROR,
339-
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
340-
errmsg("huge TLB pages not supported on this platform")));
367+
/* PGSharedMemoryCreate should have dealt with this case */
368+
Assert(huge_pages!=HUGE_PAGES_ON);
341369
#else
342370
if (huge_pages==HUGE_PAGES_ON||huge_pages==HUGE_PAGES_TRY)
343371
{
@@ -366,12 +394,12 @@ CreateAnonymousSegment(Size *size)
366394
PG_MMAP_FLAGS |MAP_HUGETLB,-1,0);
367395
mmap_errno=errno;
368396
if (huge_pages==HUGE_PAGES_TRY&&ptr==MAP_FAILED)
369-
elog(DEBUG1,"mmap with MAP_HUGETLB failed, huge pages disabled: %m");
397+
elog(DEBUG1,"mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
398+
allocsize);
370399
}
371400
#endif
372401

373-
if (huge_pages==HUGE_PAGES_OFF||
374-
(huge_pages==HUGE_PAGES_TRY&&ptr==MAP_FAILED))
402+
if (ptr==MAP_FAILED&&huge_pages!=HUGE_PAGES_ON)
375403
{
376404
/*
377405
* use the original size, not the rounded up value, when falling back
@@ -401,7 +429,25 @@ CreateAnonymousSegment(Size *size)
401429
*size=allocsize;
402430
returnptr;
403431
}
404-
#endif
432+
433+
/*
434+
* AnonymousShmemDetach --- detach from an anonymous mmap'd block
435+
* (called as an on_shmem_exit callback, hence funny argument list)
436+
*/
437+
staticvoid
438+
AnonymousShmemDetach(intstatus,Datumarg)
439+
{
440+
/* Release anonymous shared memory block, if any. */
441+
if (AnonymousShmem!=NULL)
442+
{
443+
if (munmap(AnonymousShmem,AnonymousShmemSize)<0)
444+
elog(LOG,"munmap(%p, %zu) failed: %m",
445+
AnonymousShmem,AnonymousShmemSize);
446+
AnonymousShmem=NULL;
447+
}
448+
}
449+
450+
#endif/* USE_ANONYMOUS_SHMEM */
405451

406452
/*
407453
* PGSharedMemoryCreate
@@ -432,7 +478,8 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port,
432478
structstatstatbuf;
433479
Sizesysvsize;
434480

435-
#if defined(EXEC_BACKEND)|| !defined(MAP_HUGETLB)
481+
/* Complain if hugepages demanded but we can't possibly support them */
482+
#if !defined(USE_ANONYMOUS_SHMEM)|| !defined(MAP_HUGETLB)
436483
if (huge_pages==HUGE_PAGES_ON)
437484
ereport(ERROR,
438485
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
@@ -442,32 +489,13 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port,
442489
/* Room for a header? */
443490
Assert(size>MAXALIGN(sizeof(PGShmemHeader)));
444491

445-
/*
446-
* As of PostgreSQL 9.3, we normally allocate only a very small amount of
447-
* System V shared memory, and only for the purposes of providing an
448-
* interlock to protect the data directory. The real shared memory block
449-
* is allocated using mmap(). This works around the problem that many
450-
* systems have very low limits on the amount of System V shared memory
451-
* that can be allocated. Even a limit of a few megabytes will be enough
452-
* to run many copies of PostgreSQL without needing to adjust system
453-
* settings.
454-
*
455-
* We assume that no one will attempt to run PostgreSQL 9.3 or later on
456-
* systems that are ancient enough that anonymous shared memory is not
457-
* supported, such as pre-2.4 versions of Linux. If that turns out to be
458-
* false, we might need to add a run-time test here and do this only if
459-
* the running kernel supports it.
460-
*
461-
* However, we disable this logic in the EXEC_BACKEND case, and fall back
462-
* to the old method of allocating the entire segment using System V
463-
* shared memory, because there's no way to attach an mmap'd segment to a
464-
* process after exec(). Since EXEC_BACKEND is intended only for
465-
* developer use, this shouldn't be a big problem.
466-
*/
467-
#ifndefEXEC_BACKEND
492+
#ifdefUSE_ANONYMOUS_SHMEM
468493
AnonymousShmem=CreateAnonymousSegment(&size);
469494
AnonymousShmemSize=size;
470495

496+
/* Register on-exit routine to unmap the anonymous segment */
497+
on_shmem_exit(AnonymousShmemDetach, (Datum)0);
498+
471499
/* Now we need only allocate a minimal-sized SysV shmem block. */
472500
sysvsize=sizeof(PGShmemHeader);
473501
#else
@@ -572,10 +600,14 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port,
572600
* block. Otherwise, the System V shared memory block is only a shim, and
573601
* we must return a pointer to the real block.
574602
*/
603+
#ifdefUSE_ANONYMOUS_SHMEM
575604
if (AnonymousShmem==NULL)
576605
returnhdr;
577606
memcpy(AnonymousShmem,hdr,sizeof(PGShmemHeader));
578607
return (PGShmemHeader*)AnonymousShmem;
608+
#else
609+
returnhdr;
610+
#endif
579611
}
580612

581613
#ifdefEXEC_BACKEND
@@ -660,12 +692,12 @@ PGSharedMemoryNoReAttach(void)
660692
*
661693
* Detach from the shared memory segment, if still attached. This is not
662694
* intended to be called explicitly by the process that originally created the
663-
* segment (it will haveanon_shmem_exit callback registered to do that).
695+
* segment (it will have on_shmem_exit callback(s) registered to do that).
664696
* Rather, this is for subprocesses that have inherited an attachment and want
665697
* to get rid of it.
666698
*
667699
* UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
668-
* routine.
700+
* routine, also AnonymousShmem and AnonymousShmemSize.
669701
*/
670702
void
671703
PGSharedMemoryDetach(void)
@@ -682,10 +714,15 @@ PGSharedMemoryDetach(void)
682714
UsedShmemSegAddr=NULL;
683715
}
684716

685-
/* Release anonymous shared memory block, if any. */
686-
if (AnonymousShmem!=NULL
687-
&&munmap(AnonymousShmem,AnonymousShmemSize)<0)
688-
elog(LOG,"munmap(%p) failed: %m",AnonymousShmem);
717+
#ifdefUSE_ANONYMOUS_SHMEM
718+
if (AnonymousShmem!=NULL)
719+
{
720+
if (munmap(AnonymousShmem,AnonymousShmemSize)<0)
721+
elog(LOG,"munmap(%p, %zu) failed: %m",
722+
AnonymousShmem,AnonymousShmemSize);
723+
AnonymousShmem=NULL;
724+
}
725+
#endif
689726
}
690727

691728

‎src/backend/postmaster/bgworker.c

Lines changed: 0 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -602,7 +602,6 @@ StartBackgroundWorker(void)
602602
*/
603603
if ((worker->bgw_flags&BGWORKER_SHMEM_ACCESS)==0)
604604
{
605-
on_exit_reset();
606605
dsm_detach_all();
607606
PGSharedMemoryDetach();
608607
}

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