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Re: Buffer locking is special (hints, checksums, AIO writes)

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andres Freund <andres@anarazel.de>, Melanie Plageman <melanieplageman@gmail.com>, Noah Misch <noah@leadboat.com>
Cc: Kirill Reshke <reshkekirill@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>
Date: 2026-02-07T10:44:25Z
Lists: pgsql-hackers 
On 03/02/2026 00:33, Andres Freund wrote:
> - The way MarkBufferDirtyHint() operates was copied into
>    heap_inplace_update_and_unlock(). Now that MarkBufferDirtyHint() won't work
>    that way anymore, it seems better to go with the alternative approach the
>    comments already outlined, namely to only delay updating of the buffer
>    contents.
> 
>    I've done this in a prequisite commit, as it doesn't actually depend on any
>    of the other changes.  Noah, any chance you could take a look at this?

Patch 0001 Looks correct to me. However:

> 	 * ["D" is a VACUUM (ONLY_DATABASE_STATS)]
> 	 * ["R" is a VACUUM tbl]
> 	 * D: vac_update_datfrozenxid() -> systable_beginscan(pg_class)
> 	 * D: systable_getnext() returns pg_class tuple of tbl
> 	 * R: memcpy() into pg_class tuple of tbl
> 	 * D: raise pg_database.datfrozenxid, XLogInsert(), finish
> 	 * [crash]
> 	 * [recovery restores datfrozenxid w/o relfrozenxid]
> 	 *
> 	 * As we hold an exclusive lock - preventing the buffer from being written
> 	 * out once dirty - we can work around this as follows: MarkBufferDirty(),
> 	 * XLogInsert(), memcpy().

That last reference to 'memcpy' is a little orphaned now. The comment 
used to talk about the stack copy of the page, but now there's no 
mention of that except for this reference to memcpy(). To make things 
worse, the steps have "memcpy() into pg_class tuple of tbl", so one 
could think that the "memcpy" refers to that.

How about this:

	 * We avoid that by using a temporary copy of the buffer to hide our
	 * change from other backends until it's been WAL-logged. We apply our
	 * change to the temporary copy and WAL-log it before modifying the real
	 * page. That way any action a reader of the in-place-updated value takes
	 * will be WAL logged after this change.

- Heikki

Commits

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

  1. bufmgr: Fix ordering of checks in PinBuffer()

  2. Use UnlockReleaseBuffer() in more places

  3. bufmgr: Make UnlockReleaseBuffer() more efficient

  4. bufmgr: Don't copy pages while writing out

  5. Fix use of wrong variable in _hash_kill_items()

  6. Fix bug due to confusion about what IsMVCCSnapshot means

  7. bufmgr: Switch to standard order in MarkBufferDirtyHint()

  8. bufmgr: Remove the, now obsolete, BM_JUST_DIRTIED

  9. Require share-exclusive lock to set hint bits and to flush

  10. heapam: Don't mimic MarkBufferDirtyHint() in inplace updates

  11. bufmgr: Allow conditionally locking of already locked buffer

  12. bufmgr: Avoid spurious compiler warning after fcb9c977aa5

  13. lwlock: Remove ForEachLWLockHeldByMe

  14. lwlock: Remove support for disowned lwlwocks

  15. bufmgr: Implement buffer content locks independently of lwlocks

  16. bufmgr: Change BufferDesc.state to be a 64-bit atomic

  17. lwlock: Improve local variable name

  18. lwlock: Invert meaning of LW_FLAG_RELEASE_OK

  19. bufmgr: Make definitions related to buffer descriptor easier to modify

  20. heapam: Add batch mode mvcc check and use it in page mode

  21. freespace: Don't modify page without any lock

  22. heapam: Move logic to handle HEAP_MOVED into a helper function

  23. bufmgr: Optimize & harmonize LockBufHdr(), LWLockWaitListLock()

  24. bufmgr: Add one-entry cache for private refcount

  25. bufmgr: Separate keys for private refcount infrastructure

  26. Add pg_atomic_unlocked_write_u64

  27. Rename BUFFERPIN wait event class to BUFFER

  28. bufmgr: Turn BUFFER_LOCK_* into an enum

  29. lwlock: Fix, currently harmless, bug in LWLockWakeup()

  30. bufmgr: Use atomic sub for unpinning buffers

  31. bufmgr: Allow some buffer state modifications while holding header lock

  32. bufmgr: Fix valgrind checking for buffers pinned in StrategyGetBuffer()

  33. bufmgr: Don't lock buffer header in StrategyGetBuffer()

  34. bufmgr: fewer calls to BufferDescriptorGetContentLock

  35. bufmgr: Fix signedness of mask variable in BufferSync()

  36. bufmgr: Introduce FlushUnlockedBuffer

  37. Improve ReadRecentBuffer() scalability