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Commit7ab9b2f

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Rearrange lazy_scan_heap to avoid visibility map race conditions.
We must set the visibility map bit before releasing our exclusive lockon the heap page; otherwise, someone might clear the heap page bitbefore we set the visibility map bit, leading to a situation where thevisibility map thinks the page is all-visible but it's really not.This problem has existed since 8.4, but it wasn't critical before wehad index-only scans, since the worst case scenario was that the pagewouldn't get vacuumed until the next scan_all vacuum.Along the way, a couple of minor, related improvements: (1) if wepause the heap scan to do an index vac cycle, release any visibilitymap page we're holding, since really long-running pins are not goodfor a variety of reasons; and (2) warn if we see a page that's markedall-visible in the visibility map but not on the page level, sincethat should never happen any more (it was allowed in previousreleases, but not in 9.2).
1 parent85efd5f commit7ab9b2f

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+46
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‎src/backend/commands/vacuumlazy.c

Lines changed: 46 additions & 39 deletions
Original file line numberDiff line numberDiff line change
@@ -490,6 +490,18 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
490490
if ((vacrelstats->max_dead_tuples-vacrelstats->num_dead_tuples)<MaxHeapTuplesPerPage&&
491491
vacrelstats->num_dead_tuples>0)
492492
{
493+
/*
494+
* Before beginning index vacuuming, we release any pin we may hold
495+
* on the visibility map page. This isn't necessary for correctness,
496+
* but we do it anyway to avoid holding the pin across a lengthy,
497+
* unrelated operation.
498+
*/
499+
if (BufferIsValid(vmbuffer))
500+
{
501+
ReleaseBuffer(vmbuffer);
502+
vmbuffer=InvalidBuffer;
503+
}
504+
493505
/* Log cleanup info before we touch indexes */
494506
vacuum_log_cleanup_info(onerel,vacrelstats);
495507

@@ -510,6 +522,16 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
510522
vacrelstats->num_index_scans++;
511523
}
512524

525+
/*
526+
* Pin the visibility map page in case we need to mark the page
527+
* all-visible. In most cases this will be very cheap, because we'll
528+
* already have the correct page pinned anyway. However, it's possible
529+
* that (a) next_not_all_visible_block is covered by a different VM page
530+
* than the current block or (b) we released our pin and did a cycle of
531+
* index vacuuming.
532+
*/
533+
visibilitymap_pin(onerel,blkno,&vmbuffer);
534+
513535
buf=ReadBufferExtended(onerel,MAIN_FORKNUM,blkno,
514536
RBM_NORMAL,vac_strategy);
515537

@@ -600,26 +622,15 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
600622
empty_pages++;
601623
freespace=PageGetHeapFreeSpace(page);
602624

625+
/* empty pages are always all-visible */
603626
if (!PageIsAllVisible(page))
604627
{
605628
PageSetAllVisible(page);
606629
MarkBufferDirty(buf);
630+
visibilitymap_set(onerel,blkno,InvalidXLogRecPtr,vmbuffer);
607631
}
608632

609-
LockBuffer(buf,BUFFER_LOCK_UNLOCK);
610-
611-
/* Update the visibility map */
612-
if (!all_visible_according_to_vm)
613-
{
614-
visibilitymap_pin(onerel,blkno,&vmbuffer);
615-
LockBuffer(buf,BUFFER_LOCK_SHARE);
616-
if (PageIsAllVisible(page))
617-
visibilitymap_set(onerel,blkno,InvalidXLogRecPtr,
618-
vmbuffer);
619-
LockBuffer(buf,BUFFER_LOCK_UNLOCK);
620-
}
621-
622-
ReleaseBuffer(buf);
633+
UnlockReleaseBuffer(buf);
623634
RecordPageWithFreeSpace(onerel,blkno,freespace);
624635
continue;
625636
}
@@ -834,11 +845,26 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
834845

835846
freespace=PageGetHeapFreeSpace(page);
836847

837-
/*Update the all-visible flag on the page */
838-
if (!PageIsAllVisible(page)&&all_visible)
848+
/*mark page all-visible, if appropriate */
849+
if (all_visible&&!all_visible_according_to_vm)
839850
{
840-
PageSetAllVisible(page);
841-
MarkBufferDirty(buf);
851+
if (!PageIsAllVisible(page))
852+
{
853+
PageSetAllVisible(page);
854+
MarkBufferDirty(buf);
855+
}
856+
visibilitymap_set(onerel,blkno,InvalidXLogRecPtr,vmbuffer);
857+
}
858+
859+
/*
860+
* As of PostgreSQL 9.2, the visibility map bit should never be set if
861+
* the page-level bit is clear.
862+
*/
863+
elseif (all_visible_according_to_vm&& !PageIsAllVisible(page))
864+
{
865+
elog(WARNING,"page is not marked all-visible but visibility map bit is set in relation \"%s\" page %u",
866+
relname,blkno);
867+
visibilitymap_clear(onerel,blkno,vmbuffer);
842868
}
843869

844870
/*
@@ -859,30 +885,11 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
859885
elog(WARNING,"page containing dead tuples is marked as all-visible in relation \"%s\" page %u",
860886
relname,blkno);
861887
PageClearAllVisible(page);
862-
SetBufferCommitInfoNeedsSave(buf);
863-
864-
/*
865-
* Normally, we would drop the lock on the heap page before
866-
* updating the visibility map, but since this case shouldn't
867-
* happen anyway, don't worry about that.
868-
*/
869-
visibilitymap_pin(onerel,blkno,&vmbuffer);
888+
MarkBufferDirty(buf);
870889
visibilitymap_clear(onerel,blkno,vmbuffer);
871890
}
872891

873-
LockBuffer(buf,BUFFER_LOCK_UNLOCK);
874-
875-
/* Update the visibility map */
876-
if (!all_visible_according_to_vm&&all_visible)
877-
{
878-
visibilitymap_pin(onerel,blkno,&vmbuffer);
879-
LockBuffer(buf,BUFFER_LOCK_SHARE);
880-
if (PageIsAllVisible(page))
881-
visibilitymap_set(onerel,blkno,InvalidXLogRecPtr,vmbuffer);
882-
LockBuffer(buf,BUFFER_LOCK_UNLOCK);
883-
}
884-
885-
ReleaseBuffer(buf);
892+
UnlockReleaseBuffer(buf);
886893

887894
/* Remember the location of the last page with nonremovable tuples */
888895
if (hastup)

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