Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Noah Misch <noah@leadboat.com>
Cc: pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Jelte Fennema-Nio <postgres@jeltef.nl>, Antonin Houska <ah@cybertec.at>
Date: 2025-04-03T18:19:43Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. aio: Fix assertion, clarify README

  2. aio: Fix reference to outdated name

  3. aio: Fix possible state confusions due to interrupt processing

  4. aio: Improve debug logging around waiting for IOs

  5. aio: Fix crash potential for pg_aios views due to late state update

  6. Increase BAS_BULKREAD based on effective_io_concurrency

  7. localbuf: Add Valgrind buffer access instrumentation

  8. aio: Make AIO more compatible with valgrind

  9. aio: Avoid spurious coverity warning

  10. tests: Fix incompatibility of test_aio with *_FORCE_RELEASE

  11. tests: Cope with WARNINGs during failed CREATE DB on windows

  12. aio: Add errcontext for processing I/Os for another backend

  13. aio: Add README.md explaining higher level design

  14. aio: Minor comment improvements

  15. aio: Add test_aio module

  16. aio: Add pg_aios view

  17. docs: Add acronym and glossary entries for I/O and AIO

  18. Enable IO concurrency on all systems

  19. read_stream: Introduce and use optional batchmode support

  20. docs: Reframe track_io_timing related docs as wait time

  21. bufmgr: Use AIO in StartReadBuffers()

  22. bufmgr: Implement AIO read support

  23. aio: Add WARNING result status

  24. Let caller of PageIsVerified() control ignore_checksum_failure

  25. pgstat: Allow checksum errors to be reported in critical sections

  26. Add errhint_internal()

  27. localbuf: Track pincount in BufferDesc as well

  28. aio, bufmgr: Comment fixes/improvements

  29. Fix mis-attribution of checksum failure stats to the wrong database

  30. aio: Implement support for reads in smgr/md/fd

  31. aio: Add io_method=io_uring

  32. aio: Add liburing dependency

  33. aio: Rename pgaio_io_prep_* to pgaio_io_start_*

  34. aio: Pass result of local callbacks to ->report_return

  35. aio: Be more paranoid about interrupts

  36. Redefine max_files_per_process to control additionally opened files

  37. aio: Change prefix of PgAioResultStatus values to PGAIO_RS_

  38. bufmgr: Improve stats when a buffer is read in concurrently

  39. aio: Add io_method=worker

  40. aio: Infrastructure for io_method=worker

  41. aio: Add core asynchronous I/O infrastructure

  42. aio: Basic subsystem initialization

  43. tests: Expand temp table tests to some pin related matters

  44. localbuf: Introduce FlushLocalBuffer()

  45. localbuf: Introduce TerminateLocalBufferIO()

  46. localbuf: Fix dangerous coding pattern in GetLocalVictimBuffer()

  47. localbuf: Introduce StartLocalBufferIO()

  48. localbuf: Introduce InvalidateLocalBuffer()

  49. Allow lwlocks to be disowned

  50. Make jsonb casts to scalar types translate JSON null to SQL NULL.

  51. bufmgr/smgr: Don't cross segment boundaries in StartReadBuffers()

  52. Use aux process resource owner in walsender

  53. bufmgr: Return early in ScheduleBufferTagForWriteback() if fsync=off

Hi,

On 2025-04-01 17:47:51 -0400, Andres Freund wrote:
> There are three different types of failures in the test_aio test so far:

And a fourth, visible after I enabled liburing support for skink.

https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=skink&dt=2025-04-03%2007%3A06%3A19&stg=pg_upgrade-check
(ignore the pg_upgrade and oauth failures, they're independent, I've raised
them separately)

4a)

2025-04-03 10:58:32.978 UTC [2486740][client backend][3/6:0] LOG:  short read injection point called, is enabled: 0
==2486740== VALGRINDERROR-BEGIN
==2486740== Invalid read of size 2
==2486740==    at 0x59C8AC: PageIsNew (bufpage.h:237)
==2486740==    by 0x59C8AC: PageIsVerified (bufpage.c:108)
==2486740==    by 0x567870: buffer_readv_complete_one (bufmgr.c:6873)
==2486740==    by 0x567870: buffer_readv_complete (bufmgr.c:6996)
==2486740==    by 0x567870: shared_buffer_readv_complete (bufmgr.c:7153)
==2486740==    by 0x55DDB2: pgaio_io_call_complete_shared (aio_callback.c:256)
==2486740==    by 0x55D6F1: pgaio_io_process_completion (aio.c:512)
==2486740==    by 0x55F53A: pgaio_uring_drain_locked (method_io_uring.c:370)
==2486740==    by 0x55F7B8: pgaio_uring_wait_one (method_io_uring.c:449)
==2486740==    by 0x55C702: pgaio_io_wait (aio.c:587)
==2486740==    by 0x55C8B0: pgaio_wref_wait (aio.c:900)
==2486740==    by 0x8639240: read_rel_block_ll (test_aio.c:440)
==2486740==    by 0x3B915C: ExecInterpExpr (execExprInterp.c:953)
==2486740==    by 0x3B4E4E: ExecInterpExprStillValid (execExprInterp.c:2299)
==2486740==    by 0x3F7E97: ExecEvalExprNoReturn (executor.h:445)
==2486740==  Address 0x8fa400e is in a rw- anonymous segment
==2486740==
==2486740== VALGRINDERROR-END

The reason for this is that this test unpins the buffer (from the backend's
view), before waiting for the IO. While the AIO subsystem holds a pin,
UnpinBufferNoOwner() marked the buffer as inaccessible:
		/*
		 * Mark buffer non-accessible to Valgrind.
		 *
		 * Note that the buffer may have already been marked non-accessible
		 * within access method code that enforces that buffers are only
		 * accessed while a buffer lock is held.
		 */
		VALGRIND_MAKE_MEM_NOACCESS(BufHdrGetBlock(buf), BLCKSZ);


I think to fix this we need to mark buffers as accessible around the
PageIsVerified() call in buffer_readv_complete_one(), IFF they're not pinned
by the backend.  Unfortunately, this is complicated by the fact that local
buffers do not have valgrind integration :(, so we should only do that for
local buffers, as otherwise the local buffer stays inaccessible the next time
it is pinned.


4b)

That's not all though, after getting past this failure, I see uninitialized
memory errors for reads into temporary buffers:

==3334031== VALGRINDERROR-BEGIN
==3334031== Conditional jump or move depends on uninitialised value(s)
==3334031==    at 0xD7C859: PageIsVerified (bufpage.c:108)
==3334031==    by 0xD381CA: buffer_readv_complete_one (bufmgr.c:6876)
==3334031==    by 0xD385D1: buffer_readv_complete (bufmgr.c:7002)
==3334031==    by 0xD38D2E: local_buffer_readv_complete (bufmgr.c:7210)
==3334031==    by 0xD265FA: pgaio_io_call_complete_local (aio_callback.c:306)
==3334031==    by 0xD24720: pgaio_io_reclaim (aio.c:644)
==3334031==    by 0xD24400: pgaio_io_process_completion (aio.c:521)
==3334031==    by 0xD28D3D: pgaio_uring_drain_locked (method_io_uring.c:382)
==3334031==    by 0xD2905F: pgaio_uring_wait_one (method_io_uring.c:461)
==3334031==    by 0xD245E0: pgaio_io_wait (aio.c:587)
==3334031==    by 0xD24FFE: pgaio_wref_wait (aio.c:900)
==3334031==    by 0xD2F471: WaitReadBuffers (bufmgr.c:1695)
==3334031==    by 0xD2BCF4: read_stream_next_buffer (read_stream.c:898)
==3334031==    by 0x8B4861: heap_fetch_next_buffer (heapam.c:654)
==3334031==    by 0x8B4FFA: heapgettup_pagemode (heapam.c:1016)
==3334031==    by 0x8B594F: heap_getnextslot (heapam.c:1375)
==3334031==    by 0xB28AA4: table_scan_getnextslot (tableam.h:1031)
==3334031==    by 0xB29177: SeqNext (nodeSeqscan.c:81)
==3334031==    by 0xB28F75: ExecScanFetch (execScan.h:126)
==3334031==    by 0xB28FDD: ExecScanExtended (execScan.h:170)


The reason for this one is, I think, that valgrind doesn't understand io_uring
sufficiently. Which isn't surprising, io_uring's nature of an in-memory queue
of commands is somewhat hard to intercept by tools like valgrind and rr.

The best fix for that one would, I think, be to have method_io_uring() iterate
over the IOV and mark the relevant regions as defined?  That does fix the
issue at least and does seem to make sense?   Not quite sure if we should mark
the entire IOV is efined or just the portion that was actually read - the
latter is additional fiddly code, and it's not clear it's likely to be helpful?


4c)

Unfortunately, once 4a) is addressed, the VALGRIND_MAKE_MEM_NOACCESS() after
PageIsVerified() causes the *next* read into the same buffer in an IO worker
to fail:

==3339904== Syscall param pread64(buf) points to unaddressable byte(s)
==3339904==    at 0x5B3B687: __internal_syscall_cancel (cancellation.c:64)
==3339904==    by 0x5B3B6AC: __syscall_cancel (cancellation.c:75)
==3339904==    by 0x5B93C83: pread (pread64.c:25)
==3339904==    by 0xD274F4: pg_preadv (pg_iovec.h:56)
==3339904==    by 0xD2799A: pgaio_io_perform_synchronously (aio_io.c:137)
==3339904==    by 0xD2A6D7: IoWorkerMain (method_worker.c:538)
==3339904==    by 0xC91E26: postmaster_child_launch (launch_backend.c:290)
==3339904==    by 0xC99594: StartChildProcess (postmaster.c:3972)
==3339904==    by 0xC99EE3: maybe_adjust_io_workers (postmaster.c:4403)
==3339904==    by 0xC958A8: PostmasterMain (postmaster.c:1381)
==3339904==    by 0xB69622: main (main.c:227)
==3339904==  Address 0x7f936d386000 is in a rw- anonymous segment

Because, from the view of the IO worker, that memory is still marked NOACCESS,
even if it since has been marked accessible in the backend.


We could adress this by conditioning the VALGRIND_MAKE_MEM_NOACCESS() on not
being in an IO worker, but it seems better to instead explicitly mark the
region accessible in the worker, before executing the IO.

In a first hack, I did that in pgaio_io_perform_synchronously(), but that is
likely too broad.  I don't think the same scenario exists when IOs are
executed synchronously in the foreground.


Questions:

1) It'd be cleaner to implement valgrind support in localbuf.c, so we don't
   need to have special-case logic for that. But it also makes the change less
   localized and more "impactful", who knows what kind of skullduggery we have
   been getting away with unnoticed.

   I haven't written the code up yet, but I don't think it'd be all that much
   code to add valgrind support to localbuf.

2) Any better ideas to handle the above issues than what I outlined?

Greetings,

Andres Freund