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Commit5f08acc

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Fix past pd_upper write in ginRedoRecompress()
ginRedoRecompress() replays actions over compressed segments of posting listin-place. However, it might lead to write past pg_upper, because intermediatestate during playing the changes can take more space than both original stateand final state. This commit fixes that by refuse from in-place modification.Instead page tail is copied once modification is started, and then it's usedas the source of original segments. Backpatch to 9.4 where posting listcompression was introduced.Reported-by: Sivasubramanian RamasubramanianDiscussion:https://postgr.es/m/1536091151804.6588%40amazon.comAuthor: Alexander Korotkov based on patch from and ideas by Sivasubramanian RamasubramanianReview: Sivasubramanian RamasubramanianBackpatch-through: 9.4
1 parentff47d4b commit5f08acc

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+59
-21
lines changed

1 file changed

+59
-21
lines changed

‎src/backend/access/gin/ginxlog.c‎

Lines changed: 59 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -135,6 +135,14 @@ ginRedoInsertEntry(Buffer buffer, bool isLeaf, BlockNumber rightblkno, void *rda
135135
}
136136
}
137137

138+
/*
139+
* Redo recompression of posting list. Doing all the changes in-place is not
140+
* always possible, because it might require more space than we've on the page.
141+
* Instead, once modification is required we copy unprocessed tail of the page
142+
* into separately allocated chunk of memory for further reading original
143+
* versions of segments. Thanks to that we don't bother about moving page data
144+
* in-place.
145+
*/
138146
staticvoid
139147
ginRedoRecompress(Pagepage,ginxlogRecompressDataLeaf*data)
140148
{
@@ -144,6 +152,9 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
144152
Pointersegmentend;
145153
char*walbuf;
146154
inttotalsize;
155+
PointertailCopy=NULL;
156+
PointerwritePtr;
157+
Pointersegptr;
147158

148159
/*
149160
* If the page is in pre-9.4 format, convert to new format first.
@@ -183,6 +194,7 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
183194
}
184195

185196
oldseg=GinDataLeafPageGetPostingList(page);
197+
writePtr= (Pointer)oldseg;
186198
segmentend= (Pointer)oldseg+GinDataLeafPageGetPostingListSize(page);
187199
segno=0;
188200

@@ -200,8 +212,6 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
200212
ItemPointerData*newitems;
201213
intnnewitems;
202214
intsegsize;
203-
Pointersegptr;
204-
intszleft;
205215

206216
/* Extract all the information we need from the WAL record */
207217
if (a_action==GIN_SEGMENT_INSERT||
@@ -224,6 +234,17 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
224234
Assert(segno <=a_segno);
225235
while (segno<a_segno)
226236
{
237+
/*
238+
* Once modification is started and page tail is copied, we've
239+
* to copy unmodified segments.
240+
*/
241+
segsize=SizeOfGinPostingList(oldseg);
242+
if (tailCopy)
243+
{
244+
Assert(writePtr+segsize<PageGetSpecialPointer(page));
245+
memcpy(writePtr, (Pointer)oldseg,segsize);
246+
}
247+
writePtr+=segsize;
227248
oldseg=GinNextPostingListSegment(oldseg);
228249
segno++;
229250
}
@@ -264,36 +285,42 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
264285
Assert(a_action==GIN_SEGMENT_INSERT);
265286
segsize=0;
266287
}
267-
szleft=segmentend-segptr;
288+
289+
/*
290+
* We're about to start modification of the page. So, copy tail of the
291+
* page if it's not done already.
292+
*/
293+
if (!tailCopy&&segptr!=segmentend)
294+
{
295+
inttailSize=segmentend-segptr;
296+
297+
tailCopy= (Pointer)palloc(tailSize);
298+
memcpy(tailCopy,segptr,tailSize);
299+
segptr=tailCopy;
300+
oldseg= (GinPostingList*)segptr;
301+
segmentend=segptr+tailSize;
302+
}
268303

269304
switch (a_action)
270305
{
271306
caseGIN_SEGMENT_DELETE:
272-
memmove(segptr,segptr+segsize,szleft-segsize);
273-
segmentend-=segsize;
274-
307+
segptr+=segsize;
275308
segno++;
276309
break;
277310

278311
caseGIN_SEGMENT_INSERT:
279-
/* make room for the new segment */
280-
memmove(segptr+newsegsize,segptr,szleft);
281312
/* copy the new segment in place */
282-
memcpy(segptr,newseg,newsegsize);
283-
segmentend+=newsegsize;
284-
segptr+=newsegsize;
313+
Assert(writePtr+newsegsize <=PageGetSpecialPointer(page));
314+
memcpy(writePtr,newseg,newsegsize);
315+
writePtr+=newsegsize;
285316
break;
286317

287318
caseGIN_SEGMENT_REPLACE:
288-
/* shift the segments that follow */
289-
memmove(segptr+newsegsize,
290-
segptr+segsize,
291-
szleft-segsize);
292-
/* copy the replacement segment in place */
293-
memcpy(segptr,newseg,newsegsize);
294-
segmentend-=segsize;
295-
segmentend+=newsegsize;
296-
segptr+=newsegsize;
319+
/* copy the new version of segment in place */
320+
Assert(writePtr+newsegsize <=PageGetSpecialPointer(page));
321+
memcpy(writePtr,newseg,newsegsize);
322+
writePtr+=newsegsize;
323+
segptr+=segsize;
297324
segno++;
298325
break;
299326

@@ -303,7 +330,18 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
303330
oldseg= (GinPostingList*)segptr;
304331
}
305332

306-
totalsize=segmentend- (Pointer)GinDataLeafPageGetPostingList(page);
333+
/* Copy the rest of unmodified segments if any. */
334+
segptr= (Pointer)oldseg;
335+
if (segptr!=segmentend&&tailCopy)
336+
{
337+
intrestSize=segmentend-segptr;
338+
339+
Assert(writePtr+restSize <=PageGetSpecialPointer(page));
340+
memcpy(writePtr,segptr,restSize);
341+
writePtr+=restSize;
342+
}
343+
344+
totalsize=writePtr- (Pointer)GinDataLeafPageGetPostingList(page);
307345
GinDataPageSetDataSize(page,totalsize);
308346
}
309347

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