Horton

Re: AIO v2.5

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>
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: 2025-04-03T19:40:23Z
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 aio: Fix reference to outdated name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

On Thu, Apr 03, 2025 at 02:19:43PM -0400, Andres Freund wrote:
> 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?

Makes sense.  Valgrind knows that read() makes its target bytes "defined".  It
probably doesn't have an io_uring equivalent for that.

I expect we only need this for local buffers, and it's unclear to me how the
fix for (4a) didn't fix this.  Before bufmgr Valgrind integration (1e0dfd1 of
2020-07) there was no explicit handling of shared_buffers.  I suspect that
worked because the initial mmap() of shared memory was considered "defined"
(zeros), and steps like PageAddItem() copy only defined bytes into buffers.
Hence, shared_buffers remained defined without explicit Valgrind client
requests.  This example uses local buffers.  Storage for those comes from
MemoryContextAlloc() in GetLocalBufferStorage().  That memory starts
undefined, but it becomes defined at PageInit() or read().  Hence, I expected
the fix for (4a) to make the buffer defined after io_uring read.  What makes
the outcome different?

In the general case, we could want client requests as follows:

- If completor==definer and has not dropped pin:
  - Make defined before verifying page.  That's all.  It might be cleaner to
    do this when first retrieving a return value from io_uring, since this
    just makes up for what Valgrind already does for readv().

- If completor!=definer or has dropped pin:
  - Make NOACCESS in definer when definer cedes its own pin.
  - For io_method=worker, make UNDEFINED before starting readv().  It might be
    cleanest to do this when the worker first acts as the owner of the AIO
    subsystem pin, if that's a clear moment earlier than readv().
  - Make DEFINED in completor before verifying page.  It might be cleaner to
    do this when the completor first retrieves a return value from io_uring,
    since this just makes up for what Valgrind already does for readv().
  - Make NOACCESS in completor after verifying page.  Similarly, it might be
    cleaner to do this when the completor releases the AIO subsystem pin.

> 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?

Seems fine to do the simpler way if that saves fiddly code.

> 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.

Sounds good.  Since the definer gave the AIO subsystem a pin on the worker's
behalf, it's like the worker is doing an implicit pin and explicit unpin.

> 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.

It would be the right thing long-term, and it's not a big deal if it causes
some false positives initially.  So if you're leaning that way, that's good.

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

Not here, unless the discussion under (4b) differs usefully from what you
planned.