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

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>, Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Melanie Plageman <melanieplageman@gmail.com>, 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: 2026-02-15T19:52:39Z
Lists: pgsql-hackers 
On Sat, Feb 07, 2026 at 12:44:25PM +0200, Heikki Linnakangas wrote:
> 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?

v12-0001-heapam-Don-t-mimic-MarkBufferDirtyHint-in-inplac.patch looks good.

> 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.

"memcpy" does refer to "memcpy() into pg_class tuple of tbl", so I don't see
that as orphaned.  Nonetheless:

> 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.

Either v12 or v12 w/ this edit is fine with me.  I find this proposed text
redundant with nearby comment "register block matching what buffer will look
like after changes", so I mildly prefer v12.

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