1 /*-------------------------------------------------------------------------
2 *
3 * visibilitymap.c
4 * bitmap for tracking visibility of heap tuples
5 *
6 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
7 * Portions Copyright (c) 1994, Regents of the University of California
8 *
9 *
10 * IDENTIFICATION
11 * src/backend/access/heap/visibilitymap.c
12 *
13 * INTERFACE ROUTINES
14 * visibilitymap_clear - clear bits for one page in the visibility map
15 * visibilitymap_pin - pin a map page for setting a bit
16 * visibilitymap_pin_ok - check whether correct map page is already pinned
17 * visibilitymap_set - set a bit in a previously pinned page
18 * visibilitymap_get_status - get status of bits
19 * visibilitymap_count - count number of bits set in visibility map
20 * visibilitymap_prepare_truncate -
21 * prepare for truncation of the visibility map
22 *
23 * NOTES
24 *
25 * The visibility map is a bitmap with two bits (all-visible and all-frozen)
26 * per heap page. A set all-visible bit means that all tuples on the page are
27 * known visible to all transactions, and therefore the page doesn't need to
28 * be vacuumed. A set all-frozen bit means that all tuples on the page are
29 * completely frozen, and therefore the page doesn't need to be vacuumed even
30 * if whole table scanning vacuum is required (e.g. anti-wraparound vacuum).
31 * The all-frozen bit must be set only when the page is already all-visible.
32 *
33 * The map is conservative in the sense that we make sure that whenever a bit
34 * is set, we know the condition is true, but if a bit is not set, it might or
35 * might not be true.
36 *
37 * Clearing visibility map bits is not separately WAL-logged. The callers
38 * must make sure that whenever a bit is cleared, the bit is cleared on WAL
39 * replay of the updating operation as well.
40 *
41 * When we *set* a visibility map during VACUUM, we must write WAL. This may
42 * seem counterintuitive, since the bit is basically a hint: if it is clear,
43 * it may still be the case that every tuple on the page is visible to all
44 * transactions; we just don't know that for certain. The difficulty is that
45 * there are two bits which are typically set together: the PD_ALL_VISIBLE bit
46 * on the page itself, and the visibility map bit. If a crash occurs after the
47 * visibility map page makes it to disk and before the updated heap page makes
48 * it to disk, redo must set the bit on the heap page. Otherwise, the next
49 * insert, update, or delete on the heap page will fail to realize that the
50 * visibility map bit must be cleared, possibly causing index-only scans to
51 * return wrong answers.
52 *
53 * VACUUM will normally skip pages for which the visibility map bit is set;
54 * such pages can't contain any dead tuples and therefore don't need vacuuming.
55 *
56 * LOCKING
57 *
58 * In heapam.c, whenever a page is modified so that not all tuples on the
59 * page are visible to everyone anymore, the corresponding bit in the
60 * visibility map is cleared. In order to be crash-safe, we need to do this
61 * while still holding a lock on the heap page and in the same critical
62 * section that logs the page modification. However, we don't want to hold
63 * the buffer lock over any I/O that may be required to read in the visibility
64 * map page. To avoid this, we examine the heap page before locking it;
65 * if the page-level PD_ALL_VISIBLE bit is set, we pin the visibility map
66 * bit. Then, we lock the buffer. But this creates a race condition: there
67 * is a possibility that in the time it takes to lock the buffer, the
68 * PD_ALL_VISIBLE bit gets set. If that happens, we have to unlock the
69 * buffer, pin the visibility map page, and relock the buffer. This shouldn't
70 * happen often, because only VACUUM currently sets visibility map bits,
71 * and the race will only occur if VACUUM processes a given page at almost
72 * exactly the same time that someone tries to further modify it.
73 *
74 * To set a bit, you need to hold a lock on the heap page. That prevents
75 * the race condition where VACUUM sees that all tuples on the page are
76 * visible to everyone, but another backend modifies the page before VACUUM
77 * sets the bit in the visibility map.
78 *
79 * When a bit is set, the LSN of the visibility map page is updated to make
80 * sure that the visibility map update doesn't get written to disk before the
81 * WAL record of the changes that made it possible to set the bit is flushed.
82 * But when a bit is cleared, we don't have to do that because it's always
83 * safe to clear a bit in the map from correctness point of view.
84 *
85 *-------------------------------------------------------------------------
86 */
87 #include "postgres.h"
88
89 #include "access/heapam_xlog.h"
90 #include "access/visibilitymap.h"
91 #include "access/xlog.h"
92 #include "miscadmin.h"
93 #include "port/pg_bitutils.h"
94 #include "storage/bufmgr.h"
95 #include "storage/lmgr.h"
96 #include "storage/smgr.h"
97 #include "utils/inval.h"
98
99
100 /*#define TRACE_VISIBILITYMAP */
101
102 /*
103 * Size of the bitmap on each visibility map page, in bytes. There's no
104 * extra headers, so the whole page minus the standard page header is
105 * used for the bitmap.
106 */
107 #define MAPSIZE (BLCKSZ - MAXALIGN(SizeOfPageHeaderData))
108
109 /* Number of heap blocks we can represent in one byte */
110 #define HEAPBLOCKS_PER_BYTE (BITS_PER_BYTE / BITS_PER_HEAPBLOCK)
111
112 /* Number of heap blocks we can represent in one visibility map page. */
113 #define HEAPBLOCKS_PER_PAGE (MAPSIZE * HEAPBLOCKS_PER_BYTE)
114
115 /* Mapping from heap block number to the right bit in the visibility map */
116 #define HEAPBLK_TO_MAPBLOCK(x) ((x) / HEAPBLOCKS_PER_PAGE)
117 #define HEAPBLK_TO_MAPBYTE(x) (((x) % HEAPBLOCKS_PER_PAGE) / HEAPBLOCKS_PER_BYTE)
118 #define HEAPBLK_TO_OFFSET(x) (((x) % HEAPBLOCKS_PER_BYTE) * BITS_PER_HEAPBLOCK)
119
120 /* Masks for counting subsets of bits in the visibility map. */
121 #define VISIBLE_MASK64 UINT64CONST(0x5555555555555555) /* The lower bit of each
122 * bit pair */
123 #define FROZEN_MASK64 UINT64CONST(0xaaaaaaaaaaaaaaaa) /* The upper bit of each
124 * bit pair */
125
126 /* prototypes for internal routines */
127 static Buffer vm_readbuf(Relation rel, BlockNumber blkno, bool extend);
128 static void vm_extend(Relation rel, BlockNumber vm_nblocks);
129
130
131 /*
132 * visibilitymap_clear - clear specified bits for one page in visibility map
133 *
134 * You must pass a buffer containing the correct map page to this function.
135 * Call visibilitymap_pin first to pin the right one. This function doesn't do
136 * any I/O. Returns true if any bits have been cleared and false otherwise.
137 */
138 bool
visibilitymap_clear(Relation rel,BlockNumber heapBlk,Buffer buf,uint8 flags)139 visibilitymap_clear(Relation rel, BlockNumber heapBlk, Buffer buf, uint8 flags)
140 {
141 BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
142 int mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
143 int mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
144 uint8 mask = flags << mapOffset;
145 char *map;
146 bool cleared = false;
147
148 Assert(flags & VISIBILITYMAP_VALID_BITS);
149
150 #ifdef TRACE_VISIBILITYMAP
151 elog(DEBUG1, "vm_clear %s %d", RelationGetRelationName(rel), heapBlk);
152 #endif
153
154 if (!BufferIsValid(buf) || BufferGetBlockNumber(buf) != mapBlock)
155 elog(ERROR, "wrong buffer passed to visibilitymap_clear");
156
157 LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
158 map = PageGetContents(BufferGetPage(buf));
159
160 if (map[mapByte] & mask)
161 {
162 map[mapByte] &= ~mask;
163
164 MarkBufferDirty(buf);
165 cleared = true;
166 }
167
168 LockBuffer(buf, BUFFER_LOCK_UNLOCK);
169
170 return cleared;
171 }
172
173 /*
174 * visibilitymap_pin - pin a map page for setting a bit
175 *
176 * Setting a bit in the visibility map is a two-phase operation. First, call
177 * visibilitymap_pin, to pin the visibility map page containing the bit for
178 * the heap page. Because that can require I/O to read the map page, you
179 * shouldn't hold a lock on the heap page while doing that. Then, call
180 * visibilitymap_set to actually set the bit.
181 *
182 * On entry, *buf should be InvalidBuffer or a valid buffer returned by
183 * an earlier call to visibilitymap_pin or visibilitymap_get_status on the same
184 * relation. On return, *buf is a valid buffer with the map page containing
185 * the bit for heapBlk.
186 *
187 * If the page doesn't exist in the map file yet, it is extended.
188 */
189 void
visibilitymap_pin(Relation rel,BlockNumber heapBlk,Buffer * buf)190 visibilitymap_pin(Relation rel, BlockNumber heapBlk, Buffer *buf)
191 {
192 BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
193
194 /* Reuse the old pinned buffer if possible */
195 if (BufferIsValid(*buf))
196 {
197 if (BufferGetBlockNumber(*buf) == mapBlock)
198 return;
199
200 ReleaseBuffer(*buf);
201 }
202 *buf = vm_readbuf(rel, mapBlock, true);
203 }
204
205 /*
206 * visibilitymap_pin_ok - do we already have the correct page pinned?
207 *
208 * On entry, buf should be InvalidBuffer or a valid buffer returned by
209 * an earlier call to visibilitymap_pin or visibilitymap_get_status on the same
210 * relation. The return value indicates whether the buffer covers the
211 * given heapBlk.
212 */
213 bool
visibilitymap_pin_ok(BlockNumber heapBlk,Buffer buf)214 visibilitymap_pin_ok(BlockNumber heapBlk, Buffer buf)
215 {
216 BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
217
218 return BufferIsValid(buf) && BufferGetBlockNumber(buf) == mapBlock;
219 }
220
221 /*
222 * visibilitymap_set - set bit(s) on a previously pinned page
223 *
224 * recptr is the LSN of the XLOG record we're replaying, if we're in recovery,
225 * or InvalidXLogRecPtr in normal running. The page LSN is advanced to the
226 * one provided; in normal running, we generate a new XLOG record and set the
227 * page LSN to that value. cutoff_xid is the largest xmin on the page being
228 * marked all-visible; it is needed for Hot Standby, and can be
229 * InvalidTransactionId if the page contains no tuples. It can also be set
230 * to InvalidTransactionId when a page that is already all-visible is being
231 * marked all-frozen.
232 *
233 * Caller is expected to set the heap page's PD_ALL_VISIBLE bit before calling
234 * this function. Except in recovery, caller should also pass the heap
235 * buffer. When checksums are enabled and we're not in recovery, we must add
236 * the heap buffer to the WAL chain to protect it from being torn.
237 *
238 * You must pass a buffer containing the correct map page to this function.
239 * Call visibilitymap_pin first to pin the right one. This function doesn't do
240 * any I/O.
241 */
242 void
visibilitymap_set(Relation rel,BlockNumber heapBlk,Buffer heapBuf,XLogRecPtr recptr,Buffer vmBuf,TransactionId cutoff_xid,uint8 flags)243 visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
244 XLogRecPtr recptr, Buffer vmBuf, TransactionId cutoff_xid,
245 uint8 flags)
246 {
247 BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
248 uint32 mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
249 uint8 mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
250 Page page;
251 uint8 *map;
252
253 #ifdef TRACE_VISIBILITYMAP
254 elog(DEBUG1, "vm_set %s %d", RelationGetRelationName(rel), heapBlk);
255 #endif
256
257 Assert(InRecovery || XLogRecPtrIsInvalid(recptr));
258 Assert(InRecovery || BufferIsValid(heapBuf));
259 Assert(flags & VISIBILITYMAP_VALID_BITS);
260
261 /* Check that we have the right heap page pinned, if present */
262 if (BufferIsValid(heapBuf) && BufferGetBlockNumber(heapBuf) != heapBlk)
263 elog(ERROR, "wrong heap buffer passed to visibilitymap_set");
264
265 /* Check that we have the right VM page pinned */
266 if (!BufferIsValid(vmBuf) || BufferGetBlockNumber(vmBuf) != mapBlock)
267 elog(ERROR, "wrong VM buffer passed to visibilitymap_set");
268
269 page = BufferGetPage(vmBuf);
270 map = (uint8 *) PageGetContents(page);
271 LockBuffer(vmBuf, BUFFER_LOCK_EXCLUSIVE);
272
273 if (flags != (map[mapByte] >> mapOffset & VISIBILITYMAP_VALID_BITS))
274 {
275 START_CRIT_SECTION();
276
277 map[mapByte] |= (flags << mapOffset);
278 MarkBufferDirty(vmBuf);
279
280 if (RelationNeedsWAL(rel))
281 {
282 if (XLogRecPtrIsInvalid(recptr))
283 {
284 Assert(!InRecovery);
285 recptr = log_heap_visible(rel->rd_node, heapBuf, vmBuf,
286 cutoff_xid, flags);
287
288 /*
289 * If data checksums are enabled (or wal_log_hints=on), we
290 * need to protect the heap page from being torn.
291 */
292 if (XLogHintBitIsNeeded())
293 {
294 Page heapPage = BufferGetPage(heapBuf);
295
296 /* caller is expected to set PD_ALL_VISIBLE first */
297 Assert(PageIsAllVisible(heapPage));
298 PageSetLSN(heapPage, recptr);
299 }
300 }
301 PageSetLSN(page, recptr);
302 }
303
304 END_CRIT_SECTION();
305 }
306
307 LockBuffer(vmBuf, BUFFER_LOCK_UNLOCK);
308 }
309
310 /*
311 * visibilitymap_get_status - get status of bits
312 *
313 * Are all tuples on heapBlk visible to all or are marked frozen, according
314 * to the visibility map?
315 *
316 * On entry, *buf should be InvalidBuffer or a valid buffer returned by an
317 * earlier call to visibilitymap_pin or visibilitymap_get_status on the same
318 * relation. On return, *buf is a valid buffer with the map page containing
319 * the bit for heapBlk, or InvalidBuffer. The caller is responsible for
320 * releasing *buf after it's done testing and setting bits.
321 *
322 * NOTE: This function is typically called without a lock on the heap page,
323 * so somebody else could change the bit just after we look at it. In fact,
324 * since we don't lock the visibility map page either, it's even possible that
325 * someone else could have changed the bit just before we look at it, but yet
326 * we might see the old value. It is the caller's responsibility to deal with
327 * all concurrency issues!
328 */
329 uint8
visibilitymap_get_status(Relation rel,BlockNumber heapBlk,Buffer * buf)330 visibilitymap_get_status(Relation rel, BlockNumber heapBlk, Buffer *buf)
331 {
332 BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
333 uint32 mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
334 uint8 mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
335 char *map;
336 uint8 result;
337
338 #ifdef TRACE_VISIBILITYMAP
339 elog(DEBUG1, "vm_get_status %s %d", RelationGetRelationName(rel), heapBlk);
340 #endif
341
342 /* Reuse the old pinned buffer if possible */
343 if (BufferIsValid(*buf))
344 {
345 if (BufferGetBlockNumber(*buf) != mapBlock)
346 {
347 ReleaseBuffer(*buf);
348 *buf = InvalidBuffer;
349 }
350 }
351
352 if (!BufferIsValid(*buf))
353 {
354 *buf = vm_readbuf(rel, mapBlock, false);
355 if (!BufferIsValid(*buf))
356 return false;
357 }
358
359 map = PageGetContents(BufferGetPage(*buf));
360
361 /*
362 * A single byte read is atomic. There could be memory-ordering effects
363 * here, but for performance reasons we make it the caller's job to worry
364 * about that.
365 */
366 result = ((map[mapByte] >> mapOffset) & VISIBILITYMAP_VALID_BITS);
367 return result;
368 }
369
370 /*
371 * visibilitymap_count - count number of bits set in visibility map
372 *
373 * Note: we ignore the possibility of race conditions when the table is being
374 * extended concurrently with the call. New pages added to the table aren't
375 * going to be marked all-visible or all-frozen, so they won't affect the result.
376 */
377 void
visibilitymap_count(Relation rel,BlockNumber * all_visible,BlockNumber * all_frozen)378 visibilitymap_count(Relation rel, BlockNumber *all_visible, BlockNumber *all_frozen)
379 {
380 BlockNumber mapBlock;
381 BlockNumber nvisible = 0;
382 BlockNumber nfrozen = 0;
383
384 /* all_visible must be specified */
385 Assert(all_visible);
386
387 for (mapBlock = 0;; mapBlock++)
388 {
389 Buffer mapBuffer;
390 uint64 *map;
391 int i;
392
393 /*
394 * Read till we fall off the end of the map. We assume that any extra
395 * bytes in the last page are zeroed, so we don't bother excluding
396 * them from the count.
397 */
398 mapBuffer = vm_readbuf(rel, mapBlock, false);
399 if (!BufferIsValid(mapBuffer))
400 break;
401
402 /*
403 * We choose not to lock the page, since the result is going to be
404 * immediately stale anyway if anyone is concurrently setting or
405 * clearing bits, and we only really need an approximate value.
406 */
407 map = (uint64 *) PageGetContents(BufferGetPage(mapBuffer));
408
409 StaticAssertStmt(MAPSIZE % sizeof(uint64) == 0,
410 "unsupported MAPSIZE");
411 if (all_frozen == NULL)
412 {
413 for (i = 0; i < MAPSIZE / sizeof(uint64); i++)
414 nvisible += pg_popcount64(map[i] & VISIBLE_MASK64);
415 }
416 else
417 {
418 for (i = 0; i < MAPSIZE / sizeof(uint64); i++)
419 {
420 nvisible += pg_popcount64(map[i] & VISIBLE_MASK64);
421 nfrozen += pg_popcount64(map[i] & FROZEN_MASK64);
422 }
423 }
424
425 ReleaseBuffer(mapBuffer);
426 }
427
428 *all_visible = nvisible;
429 if (all_frozen)
430 *all_frozen = nfrozen;
431 }
432
433 /*
434 * visibilitymap_prepare_truncate -
435 * prepare for truncation of the visibility map
436 *
437 * nheapblocks is the new size of the heap.
438 *
439 * Return the number of blocks of new visibility map.
440 * If it's InvalidBlockNumber, there is nothing to truncate;
441 * otherwise the caller is responsible for calling smgrtruncate()
442 * to truncate the visibility map pages.
443 */
444 BlockNumber
visibilitymap_prepare_truncate(Relation rel,BlockNumber nheapblocks)445 visibilitymap_prepare_truncate(Relation rel, BlockNumber nheapblocks)
446 {
447 BlockNumber newnblocks;
448
449 /* last remaining block, byte, and bit */
450 BlockNumber truncBlock = HEAPBLK_TO_MAPBLOCK(nheapblocks);
451 uint32 truncByte = HEAPBLK_TO_MAPBYTE(nheapblocks);
452 uint8 truncOffset = HEAPBLK_TO_OFFSET(nheapblocks);
453
454 #ifdef TRACE_VISIBILITYMAP
455 elog(DEBUG1, "vm_truncate %s %d", RelationGetRelationName(rel), nheapblocks);
456 #endif
457
458 RelationOpenSmgr(rel);
459
460 /*
461 * If no visibility map has been created yet for this relation, there's
462 * nothing to truncate.
463 */
464 if (!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
465 return InvalidBlockNumber;
466
467 /*
468 * Unless the new size is exactly at a visibility map page boundary, the
469 * tail bits in the last remaining map page, representing truncated heap
470 * blocks, need to be cleared. This is not only tidy, but also necessary
471 * because we don't get a chance to clear the bits if the heap is extended
472 * again.
473 */
474 if (truncByte != 0 || truncOffset != 0)
475 {
476 Buffer mapBuffer;
477 Page page;
478 char *map;
479
480 newnblocks = truncBlock + 1;
481
482 mapBuffer = vm_readbuf(rel, truncBlock, false);
483 if (!BufferIsValid(mapBuffer))
484 {
485 /* nothing to do, the file was already smaller */
486 return InvalidBlockNumber;
487 }
488
489 page = BufferGetPage(mapBuffer);
490 map = PageGetContents(page);
491
492 LockBuffer(mapBuffer, BUFFER_LOCK_EXCLUSIVE);
493
494 /* NO EREPORT(ERROR) from here till changes are logged */
495 START_CRIT_SECTION();
496
497 /* Clear out the unwanted bytes. */
498 MemSet(&map[truncByte + 1], 0, MAPSIZE - (truncByte + 1));
499
500 /*----
501 * Mask out the unwanted bits of the last remaining byte.
502 *
503 * ((1 << 0) - 1) = 00000000
504 * ((1 << 1) - 1) = 00000001
505 * ...
506 * ((1 << 6) - 1) = 00111111
507 * ((1 << 7) - 1) = 01111111
508 *----
509 */
510 map[truncByte] &= (1 << truncOffset) - 1;
511
512 /*
513 * Truncation of a relation is WAL-logged at a higher-level, and we
514 * will be called at WAL replay. But if checksums are enabled, we need
515 * to still write a WAL record to protect against a torn page, if the
516 * page is flushed to disk before the truncation WAL record. We cannot
517 * use MarkBufferDirtyHint here, because that will not dirty the page
518 * during recovery.
519 */
520 MarkBufferDirty(mapBuffer);
521 if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
522 log_newpage_buffer(mapBuffer, false);
523
524 END_CRIT_SECTION();
525
526 UnlockReleaseBuffer(mapBuffer);
527 }
528 else
529 newnblocks = truncBlock;
530
531 if (smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM) <= newnblocks)
532 {
533 /* nothing to do, the file was already smaller than requested size */
534 return InvalidBlockNumber;
535 }
536
537 return newnblocks;
538 }
539
540 /*
541 * Read a visibility map page.
542 *
543 * If the page doesn't exist, InvalidBuffer is returned, or if 'extend' is
544 * true, the visibility map file is extended.
545 */
546 static Buffer
vm_readbuf(Relation rel,BlockNumber blkno,bool extend)547 vm_readbuf(Relation rel, BlockNumber blkno, bool extend)
548 {
549 Buffer buf;
550
551 /*
552 * We might not have opened the relation at the smgr level yet, or we
553 * might have been forced to close it by a sinval message. The code below
554 * won't necessarily notice relation extension immediately when extend =
555 * false, so we rely on sinval messages to ensure that our ideas about the
556 * size of the map aren't too far out of date.
557 */
558 RelationOpenSmgr(rel);
559
560 /*
561 * If we haven't cached the size of the visibility map fork yet, check it
562 * first.
563 */
564 if (rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber)
565 {
566 if (smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
567 smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
568 else
569 rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = 0;
570 }
571
572 /* Handle requests beyond EOF */
573 if (blkno >= rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM])
574 {
575 if (extend)
576 vm_extend(rel, blkno + 1);
577 else
578 return InvalidBuffer;
579 }
580
581 /*
582 * Use ZERO_ON_ERROR mode, and initialize the page if necessary. It's
583 * always safe to clear bits, so it's better to clear corrupt pages than
584 * error out.
585 *
586 * The initialize-the-page part is trickier than it looks, because of the
587 * possibility of multiple backends doing this concurrently, and our
588 * desire to not uselessly take the buffer lock in the normal path where
589 * the page is OK. We must take the lock to initialize the page, so
590 * recheck page newness after we have the lock, in case someone else
591 * already did it. Also, because we initially check PageIsNew with no
592 * lock, it's possible to fall through and return the buffer while someone
593 * else is still initializing the page (i.e., we might see pd_upper as set
594 * but other page header fields are still zeroes). This is harmless for
595 * callers that will take a buffer lock themselves, but some callers
596 * inspect the page without any lock at all. The latter is OK only so
597 * long as it doesn't depend on the page header having correct contents.
598 * Current usage is safe because PageGetContents() does not require that.
599 */
600 buf = ReadBufferExtended(rel, VISIBILITYMAP_FORKNUM, blkno,
601 RBM_ZERO_ON_ERROR, NULL);
602 if (PageIsNew(BufferGetPage(buf)))
603 {
604 LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
605 if (PageIsNew(BufferGetPage(buf)))
606 PageInit(BufferGetPage(buf), BLCKSZ, 0);
607 LockBuffer(buf, BUFFER_LOCK_UNLOCK);
608 }
609 return buf;
610 }
611
612 /*
613 * Ensure that the visibility map fork is at least vm_nblocks long, extending
614 * it if necessary with zeroed pages.
615 */
616 static void
vm_extend(Relation rel,BlockNumber vm_nblocks)617 vm_extend(Relation rel, BlockNumber vm_nblocks)
618 {
619 BlockNumber vm_nblocks_now;
620 PGAlignedBlock pg;
621
622 PageInit((Page) pg.data, BLCKSZ, 0);
623
624 /*
625 * We use the relation extension lock to lock out other backends trying to
626 * extend the visibility map at the same time. It also locks out extension
627 * of the main fork, unnecessarily, but extending the visibility map
628 * happens seldom enough that it doesn't seem worthwhile to have a
629 * separate lock tag type for it.
630 *
631 * Note that another backend might have extended or created the relation
632 * by the time we get the lock.
633 */
634 LockRelationForExtension(rel, ExclusiveLock);
635
636 /* Might have to re-open if a cache flush happened */
637 RelationOpenSmgr(rel);
638
639 /*
640 * Create the file first if it doesn't exist. If smgr_vm_nblocks is
641 * positive then it must exist, no need for an smgrexists call.
642 */
643 if ((rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == 0 ||
644 rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber) &&
645 !smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
646 smgrcreate(rel->rd_smgr, VISIBILITYMAP_FORKNUM, false);
647
648 /* Invalidate cache so that smgrnblocks() asks the kernel. */
649 rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = InvalidBlockNumber;
650 vm_nblocks_now = smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
651
652 /* Now extend the file */
653 while (vm_nblocks_now < vm_nblocks)
654 {
655 PageSetChecksumInplace((Page) pg.data, vm_nblocks_now);
656
657 smgrextend(rel->rd_smgr, VISIBILITYMAP_FORKNUM, vm_nblocks_now,
658 pg.data, false);
659 vm_nblocks_now++;
660 }
661
662 /*
663 * Send a shared-inval message to force other backends to close any smgr
664 * references they may have for this rel, which we are about to change.
665 * This is a useful optimization because it means that backends don't have
666 * to keep checking for creation or extension of the file, which happens
667 * infrequently.
668 */
669 CacheInvalidateSmgr(rel->rd_smgr->smgr_rnode);
670
671 UnlockRelationForExtension(rel, ExclusiveLock);
672 }
673