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Commitdd951dc

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Prevent GIN deleted pages from being reclaimed too early
When GIN vacuum deletes a posting tree page, it assumes that no concurrentsearchers can access it, thanks to ginStepRight() locking two pages at once.However, since 9.4 searches can skip parts of posting trees descending from theroot. That leads to the risk that page is deleted and reclaimed beforeconcurrent search can access it.This commit prevents the risk of above by waiting for every transaction, whichmight wait to reference this page, to finish. Due to binary compatibilitywe can't change GinPageOpaqueData to store corresponding transaction id.Instead we reuse page header pd_prune_xid field, which is unused in index pages.Discussion:https://postgr.es/m/31a702a.14dd.166c1366ac1.Coremail.chjischj%40163.comAuthor: Andrey Borodin, Alexander KorotkovReviewed-by: Alexander KorotkovBackpatch-through: 9.4
1 parent225b5c9 commitdd951dc

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6 files changed

+22
-13
lines changed

6 files changed

+22
-13
lines changed

‎src/backend/access/gin/README

Lines changed: 4 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -304,12 +304,10 @@ the lock on next page has been acquired.
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The downlink is more tricky. A search descending the tree must release the
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lock on the parent page before locking the child, or it could deadlock with
306306
a concurrent split of the child page; a page split locks the parent, while
307-
already holding a lock on the child page. However, posting trees are only
308-
fully searched from left to right, starting from the leftmost leaf. (The
309-
tree-structure is only needed by insertions, to quickly find the correct
310-
insert location). So as long as we don't delete the leftmost page on each
311-
level, a search can never follow a downlink to page that's about to be
312-
deleted.
307+
already holding a lock on the child page. So, deleted page cannot be reclaimed
308+
immediately. Instead, we have to wait for every transaction, which might wait
309+
to reference this page, to finish. Corresponding processes must observe that
310+
the page is marked deleted and recover accordingly.
313311

314312
The previous paragraph's reasoning only applies to searches, and only to
315313
posting trees. To protect from inserters following a downlink to a deleted

‎src/backend/access/gin/ginutil.c

Lines changed: 1 addition & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -309,12 +309,7 @@ GinNewBuffer(Relation index)
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*/
310310
if (ConditionalLockBuffer(buffer))
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{
312-
Pagepage=BufferGetPage(buffer);
313-
314-
if (PageIsNew(page))
315-
returnbuffer;/* OK to use, if never initialized */
316-
317-
if (GinPageIsDeleted(page))
312+
if (GinPageIsRecyclable(BufferGetPage(buffer)))
318313
returnbuffer;/* OK to use */
319314

320315
LockBuffer(buffer,GIN_UNLOCK);

‎src/backend/access/gin/ginvacuum.c

Lines changed: 5 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -157,6 +157,9 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
157157
page=BufferGetPage(dBuffer);
158158
rightlink=GinPageGetOpaque(page)->rightlink;
159159

160+
/* For deleted page remember last xid which could knew its address */
161+
GinPageSetDeleteXid(page,ReadNewTransactionId());
162+
160163
/*
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* Any insert which would have gone on the leaf block will now go to its
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* right sibling.
@@ -213,6 +216,7 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
213216

214217
data.parentOffset=myoff;
215218
data.rightLink=GinPageGetOpaque(page)->rightlink;
219+
data.deleteXid=GinPageGetDeleteXid(page);
216220

217221
XLogRegisterData((char*)&data,sizeof(ginxlogDeletePage));
218222

@@ -732,7 +736,7 @@ ginvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
732736
LockBuffer(buffer,GIN_SHARE);
733737
page= (Page)BufferGetPage(buffer);
734738

735-
if (PageIsNew(page)||GinPageIsDeleted(page))
739+
if (GinPageIsRecyclable(page))
736740
{
737741
Assert(blkno!=GIN_ROOT_BLKNO);
738742
RecordFreeIndexPage(index,blkno);

‎src/backend/access/gin/ginxlog.c

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -531,6 +531,7 @@ ginRedoDeletePage(XLogReaderState *record)
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page=BufferGetPage(dbuffer);
532532
Assert(GinPageIsData(page));
533533
GinPageGetOpaque(page)->flags=GIN_DELETED;
534+
GinPageSetDeleteXid(page,data->deleteXid);
534535
PageSetLSN(page,lsn);
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MarkBufferDirty(dbuffer);
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}

‎src/include/access/ginblock.h

Lines changed: 10 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -10,6 +10,7 @@
1010
#ifndefGINBLOCK_H
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#defineGINBLOCK_H
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13+
#include"access/transam.h"
1314
#include"storage/block.h"
1415
#include"storage/itemptr.h"
1516
#include"storage/off.h"
@@ -127,6 +128,15 @@ typedef struct GinMetaPageData
127128

128129
#defineGinPageRightMost(page) ( GinPageGetOpaque(page)->rightlink == InvalidBlockNumber)
129130

131+
/*
132+
* We should reclaim deleted page only once every transaction started before
133+
* its deletion is over.
134+
*/
135+
#defineGinPageGetDeleteXid(page) ( ((PageHeader) (page))->pd_prune_xid )
136+
#defineGinPageSetDeleteXid(page,xid) ( ((PageHeader) (page))->pd_prune_xid = xid)
137+
#defineGinPageIsRecyclable(page) ( PageIsNew(page) || (GinPageIsDeleted(page) \
138+
&& TransactionIdPrecedes(GinPageGetDeleteXid(page), RecentGlobalXmin)))
139+
130140
/*
131141
* We use our own ItemPointerGet(BlockNumber|OffsetNumber)
132142
* to avoid Asserts, since sometimes the ip_posid isn't "valid"

‎src/include/access/ginxlog.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -158,6 +158,7 @@ typedef struct ginxlogDeletePage
158158
{
159159
OffsetNumberparentOffset;
160160
BlockNumberrightLink;
161+
TransactionIddeleteXid;/* last Xid which could see this page in scan */
161162
}ginxlogDeletePage;
162163

163164
#defineXLOG_GIN_UPDATE_META_PAGE 0x60

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