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Update FSM after updating VM on-access

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b46e1e54d0 allowed setting the VM while on-access pruning, but it
neglected to update the freespace map. Once the page was all-visible,
vacuum could skip it, leading to stale freespace map values and,
effectively, bloat. Fix it by updating the FSM if we updated the VM.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/flat/CAAKRu_b2StZrEC%3DHmW8LePuQbczyFRnfs8qTAJwn_%3DW76-y24w%40mail.gmail.com
Backpatch-through: 19
This commit is contained in:
Melanie Plageman 2026-07-10 18:10:23 -04:00
parent a1b74b7279
commit e9eaeb0424
1 changed files with 23 additions and 3 deletions
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26
src/backend/access/heap/pruneheap.c
View file

@ -27,6 +27,7 @@
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
@ -321,6 +322,9 @@ heap_page_prune_opt(Relation relation, Buffer buffer, Buffer *vmbuffer,
if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
{
bool record_free_space = false;
Size freespace = 0;
/* OK, try to get exclusive buffer lock */
if (!ConditionalLockBufferForCleanup(buffer))
return;
@ -376,16 +380,32 @@ heap_page_prune_opt(Relation relation, Buffer buffer, Buffer *vmbuffer,
if (presult.ndeleted > presult.nnewlpdead)
pgstat_update_heap_dead_tuples(relation,
presult.ndeleted - presult.nnewlpdead);
/*
* If this prune newly set the page all-visible, VACUUM may later
* skip the page and not update the free space map (FSM) for it.
* Keep the FSM from going stale by recording it now. We do not
* want to update the freespace map otherwise, to reserve
* freespace on this page for HOT updates.
*/
if (presult.newly_all_visible)
{
record_free_space = true;
freespace = PageGetHeapFreeSpace(page);
}
}
/* And release buffer lock */
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
/*
* We avoid reuse of any free space created on the page by unrelated
* UPDATEs/INSERTs by opting to not update the FSM at this point. The
* free space should be reused by UPDATEs to *this* page.
* RecordPageWithFreeSpace() only dirties the FSM when the recorded
* free-space category actually changes. Note that vacuum will still
* do FreeSpaceMapVacuum() for ranges of pages that are skipped, so we
* don't have to worry about that here.
*/
if (record_free_space)
RecordPageWithFreeSpace(relation, BufferGetBlockNumber(buffer), freespace);
}
}