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1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * visibilitymap.c
4 : : * bitmap for tracking visibility of heap tuples
5 : : *
6 : : * Portions Copyright (c) 1996-2026, 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 bit(s) 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 : : * Changes to the visibility map bits are not separately WAL-logged. Callers
38 : : * must make sure that whenever a visibility map bit is cleared, the bit is
39 : : * cleared on WAL replay of the updating operation. And whenever a visibility
40 : : * map bit is set, the bit is set on WAL replay of the operation that rendered
41 : : * the page all-visible/all-frozen.
42 : : *
43 : : * The visibility map bits operate as a hint in one direction: if they are
44 : : * clear, it may still be the case that every tuple on the page is visible to
45 : : * all transactions (we just don't know that for certain). However, if they
46 : : * are set, we may skip vacuuming pages and advance relfrozenxid or skip
47 : : * reading heap pages for an index-only scan. If they are incorrectly set,
48 : : * this can lead to data corruption and wrong results.
49 : : *
50 : : * Additionally, it is critical that the heap-page level PD_ALL_VISIBLE bit be
51 : : * correctly set and cleared along with the VM bits.
52 : : *
53 : : * When clearing the VM, if a crash occurs after the heap page makes it to
54 : : * disk but before the VM page makes it to disk, replay must clear the VM or
55 : : * the next index-only scan can return wrong results or vacuum may incorrectly
56 : : * advance relfrozenxid.
57 : : *
58 : : * When setting the VM, if a crash occurs after the visibility map page makes
59 : : * it to disk and before the updated heap page makes it to disk, redo must set
60 : : * the bit on the heap page. Otherwise, the next insert, update, or delete on
61 : : * the heap page will fail to realize that the visibility map bit must be
62 : : * cleared, possibly causing index-only scans to return wrong answers.
63 : : *
64 : : * VACUUM will normally skip pages for which the visibility map bit is set;
65 : : * such pages can't contain any dead tuples and therefore don't need vacuuming.
66 : : *
67 : : * LOCKING
68 : : *
69 : : * In heapam.c, whenever a page is modified so that not all tuples on the
70 : : * page are visible to everyone anymore, the corresponding bit in the
71 : : * visibility map is cleared. In order to be crash-safe, we need to do this
72 : : * while still holding a lock on the heap page and in the same critical
73 : : * section that logs the page modification. However, we don't want to hold
74 : : * the buffer lock over any I/O that may be required to read in the visibility
75 : : * map page. To avoid this, we examine the heap page before locking it;
76 : : * if the page-level PD_ALL_VISIBLE bit is set, we pin the visibility map
77 : : * bit. Then, we lock the buffer. But this creates a race condition: there
78 : : * is a possibility that in the time it takes to lock the buffer, the
79 : : * PD_ALL_VISIBLE bit gets set. If that happens, we have to unlock the
80 : : * buffer, pin the visibility map page, and relock the buffer. This shouldn't
81 : : * happen often, because only VACUUM currently sets visibility map bits,
82 : : * and the race will only occur if VACUUM processes a given page at almost
83 : : * exactly the same time that someone tries to further modify it.
84 : : *
85 : : * To set a bit, you need to hold a lock on the heap page. That prevents
86 : : * the race condition where VACUUM sees that all tuples on the page are
87 : : * visible to everyone, but another backend modifies the page before VACUUM
88 : : * sets the bit in the visibility map.
89 : : *
90 : : * When a bit is set, the LSN of the visibility map page is updated to make
91 : : * sure that the visibility map update doesn't get written to disk before the
92 : : * WAL record of the changes that made it possible to set the bit is flushed.
93 : : * But when a bit is cleared, we don't have to do that because it's always
94 : : * safe to clear a bit in the map from correctness point of view.
95 : : *
96 : : *-------------------------------------------------------------------------
97 : : */
98 : : #include "postgres.h"
99 : :
100 : : #include "access/heapam_xlog.h"
101 : : #include "access/visibilitymap.h"
102 : : #include "access/xloginsert.h"
103 : : #include "access/xlogutils.h"
104 : : #include "miscadmin.h"
105 : : #include "port/pg_bitutils.h"
106 : : #include "storage/bufmgr.h"
107 : : #include "storage/smgr.h"
108 : : #include "utils/inval.h"
109 : : #include "utils/rel.h"
110 : :
111 : :
112 : : /*#define TRACE_VISIBILITYMAP */
113 : :
114 : : /*
115 : : * Size of the bitmap on each visibility map page, in bytes. There's no
116 : : * extra headers, so the whole page minus the standard page header is
117 : : * used for the bitmap.
118 : : */
119 : : #define MAPSIZE (BLCKSZ - MAXALIGN(SizeOfPageHeaderData))
120 : :
121 : : /* Number of heap blocks we can represent in one byte */
122 : : #define HEAPBLOCKS_PER_BYTE (BITS_PER_BYTE / BITS_PER_HEAPBLOCK)
123 : :
124 : : /* Number of heap blocks we can represent in one visibility map page. */
125 : : #define HEAPBLOCKS_PER_PAGE (MAPSIZE * HEAPBLOCKS_PER_BYTE)
126 : :
127 : : /* Mapping from heap block number to the right bit in the visibility map */
128 : : #define HEAPBLK_TO_MAPBLOCK(x) ((x) / HEAPBLOCKS_PER_PAGE)
129 : : #define HEAPBLK_TO_MAPBLOCK_LIMIT(x) \
130 : : (((x) + HEAPBLOCKS_PER_PAGE - 1) / HEAPBLOCKS_PER_PAGE)
131 : : #define HEAPBLK_TO_MAPBYTE(x) (((x) % HEAPBLOCKS_PER_PAGE) / HEAPBLOCKS_PER_BYTE)
132 : : #define HEAPBLK_TO_OFFSET(x) (((x) % HEAPBLOCKS_PER_BYTE) * BITS_PER_HEAPBLOCK)
133 : :
134 : : /* Masks for counting subsets of bits in the visibility map. */
135 : : #define VISIBLE_MASK8 (0x55) /* The lower bit of each bit pair */
136 : : #define FROZEN_MASK8 (0xaa) /* The upper bit of each bit pair */
137 : :
138 : : /* prototypes for internal routines */
139 : : static Buffer vm_readbuf(Relation rel, BlockNumber blkno, bool extend);
140 : : static Buffer vm_extend(Relation rel, BlockNumber vm_nblocks);
141 : :
142 : : /*
143 : : * visibilitymap_clear - clear specified bits for one page in visibility map
144 : : *
145 : : * You must pass a buffer containing the correct map page to this function,
146 : : * which already needs to be pinned and locked exclusively.
147 : : *
148 : : * This function doesn't do any I/O. Returns true if any bits have been
149 : : * cleared and false otherwise.
150 : : */
151 : : bool
152 : 25701 : visibilitymap_clear(RelFileLocator rlocator, BlockNumber heapBlk,
153 : : Buffer vmbuf, uint8 flags)
154 : : {
155 : 25701 : int mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
156 : 25701 : int mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
157 : 25701 : BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
158 : 25701 : uint8 mask = flags << mapOffset;
159 : : Page page;
160 : : char *map;
161 : 25701 : bool cleared = false;
162 : :
163 : : /* Must never clear all_visible bit while leaving all_frozen bit set */
164 : : Assert(flags & VISIBILITYMAP_VALID_BITS);
165 : : Assert(flags != VISIBILITYMAP_ALL_VISIBLE);
166 : :
167 : : #ifdef TRACE_VISIBILITYMAP
168 : : elog(DEBUG1, "vm_clear %s %d",
169 : : relpathbackend(rlocator, MyProcNumber, MAIN_FORKNUM).str,
170 : : heapBlk);
171 : : #endif
172 : :
173 [ + - - + ]: 25701 : if (!BufferIsValid(vmbuf) || BufferGetBlockNumber(vmbuf) != mapBlock)
174 [ # # ]: 0 : elog(ERROR, "wrong buffer passed to visibilitymap_clear");
175 : :
176 : : Assert(BufferIsLockedByMeInMode(vmbuf, BUFFER_LOCK_EXCLUSIVE));
177 : :
178 : 25701 : page = BufferGetPage(vmbuf);
179 : 25701 : map = PageGetContents(page);
180 : :
181 [ + + ]: 25701 : if (map[mapByte] & mask)
182 : : {
183 : 21555 : map[mapByte] &= ~mask;
184 : :
185 : 21555 : MarkBufferDirty(vmbuf);
186 : 21555 : cleared = true;
187 : : }
188 : :
189 : 25701 : return cleared;
190 : : }
191 : :
192 : : /*
193 : : * visibilitymap_pin - pin a map page for setting a bit
194 : : *
195 : : * Setting a bit in the visibility map is a two-phase operation. First, call
196 : : * visibilitymap_pin, to pin the visibility map page containing the bit for
197 : : * the heap page. Because that can require I/O to read the map page, you
198 : : * shouldn't hold a lock on the heap page while doing that. Then, call
199 : : * visibilitymap_set to actually set the bit.
200 : : *
201 : : * On entry, *vmbuf should be InvalidBuffer or a valid buffer returned by
202 : : * an earlier call to visibilitymap_pin or visibilitymap_get_status on the same
203 : : * relation. On return, *vmbuf is a valid buffer with the map page containing
204 : : * the bit for heapBlk.
205 : : *
206 : : * If the page doesn't exist in the map file yet, it is extended.
207 : : */
208 : : void
209 : 1095698 : visibilitymap_pin(Relation rel, BlockNumber heapBlk, Buffer *vmbuf)
210 : : {
211 : 1095698 : BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
212 : :
213 : : /* Reuse the old pinned buffer if possible */
214 [ + + ]: 1095698 : if (BufferIsValid(*vmbuf))
215 : : {
216 [ + - ]: 552259 : if (BufferGetBlockNumber(*vmbuf) == mapBlock)
217 : 552259 : return;
218 : :
219 : 0 : ReleaseBuffer(*vmbuf);
220 : : }
221 : 543439 : *vmbuf = vm_readbuf(rel, mapBlock, true);
222 : : }
223 : :
224 : : /*
225 : : * visibilitymap_pin_ok - do we already have the correct page pinned?
226 : : *
227 : : * On entry, vmbuf should be InvalidBuffer or a valid buffer returned by
228 : : * an earlier call to visibilitymap_pin or visibilitymap_get_status on the same
229 : : * relation. The return value indicates whether the buffer covers the
230 : : * given heapBlk.
231 : : */
232 : : bool
233 : 18496 : visibilitymap_pin_ok(BlockNumber heapBlk, Buffer vmbuf)
234 : : {
235 : 18496 : BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
236 : :
237 [ + + + - ]: 18496 : return BufferIsValid(vmbuf) && BufferGetBlockNumber(vmbuf) == mapBlock;
238 : : }
239 : :
240 : : /*
241 : : * Set VM (visibility map) flags in the VM block in vmBuf.
242 : : *
243 : : * This function is intended for callers that log VM changes together
244 : : * with the heap page modifications that rendered the page all-visible.
245 : : *
246 : : * vmBuf must be pinned and exclusively locked, and it must cover the VM bits
247 : : * corresponding to heapBlk.
248 : : *
249 : : * In normal operation (not recovery), this must be called inside a critical
250 : : * section that also applies the necessary heap page changes and, if
251 : : * applicable, emits WAL.
252 : : *
253 : : * The caller is responsible for ensuring consistency between the heap page
254 : : * and the VM page by holding a pin and exclusive lock on the buffer
255 : : * containing heapBlk.
256 : : *
257 : : * rlocator is used only for debugging messages.
258 : : *
259 : : * Returns the state of the page's VM bits before setting flags.
260 : : */
261 : : uint8
262 : 91994 : visibilitymap_set(BlockNumber heapBlk,
263 : : Buffer vmBuf, uint8 flags,
264 : : RelFileLocator rlocator)
265 : : {
266 : 91994 : BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
267 : 91994 : uint32 mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
268 : 91994 : uint8 mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
269 : : Page page;
270 : : uint8 *map;
271 : : uint8 status;
272 : :
273 : : #ifdef TRACE_VISIBILITYMAP
274 : : elog(DEBUG1, "vm_set flags 0x%02X for %s %d",
275 : : flags,
276 : : relpathbackend(rlocator, MyProcNumber, MAIN_FORKNUM).str,
277 : : heapBlk);
278 : : #endif
279 : :
280 : : /* Call in same critical section where WAL is emitted. */
281 : : Assert(InRecovery || CritSectionCount > 0);
282 : :
283 : : /* Flags should be valid. Also never clear bits with this function */
284 : : Assert((flags & VISIBILITYMAP_VALID_BITS) == flags);
285 : :
286 : : /* Must never set all_frozen bit without also setting all_visible bit */
287 : : Assert(flags != VISIBILITYMAP_ALL_FROZEN);
288 : :
289 : : /* Check that we have the right VM page pinned */
290 [ + - - + ]: 91994 : if (!BufferIsValid(vmBuf) || BufferGetBlockNumber(vmBuf) != mapBlock)
291 [ # # ]: 0 : elog(ERROR, "wrong VM buffer passed to visibilitymap_set");
292 : :
293 : : Assert(BufferIsLockedByMeInMode(vmBuf, BUFFER_LOCK_EXCLUSIVE));
294 : :
295 : 91994 : page = BufferGetPage(vmBuf);
296 : 91994 : map = (uint8 *) PageGetContents(page);
297 : :
298 : 91994 : status = (map[mapByte] >> mapOffset) & VISIBILITYMAP_VALID_BITS;
299 [ + - ]: 91994 : if (flags != status)
300 : : {
301 : 91994 : map[mapByte] |= (flags << mapOffset);
302 : 91994 : MarkBufferDirty(vmBuf);
303 : : }
304 : :
305 : 91994 : return status;
306 : : }
307 : :
308 : : /*
309 : : * visibilitymap_get_status - get status of bits
310 : : *
311 : : * Are all tuples on heapBlk visible to all or are marked frozen, according
312 : : * to the visibility map?
313 : : *
314 : : * On entry, *vmbuf should be InvalidBuffer or a valid buffer returned by an
315 : : * earlier call to visibilitymap_pin or visibilitymap_get_status on the same
316 : : * relation. On return, *vmbuf is a valid buffer with the map page containing
317 : : * the bit for heapBlk, or InvalidBuffer. The caller is responsible for
318 : : * releasing *vmbuf after it's done testing and setting bits.
319 : : *
320 : : * NOTE: This function is typically called without a lock on the heap page,
321 : : * so somebody else could change the bit just after we look at it. In fact,
322 : : * since we don't lock the visibility map page either, it's even possible that
323 : : * someone else could have changed the bit just before we look at it, but yet
324 : : * we might see the old value. It is the caller's responsibility to deal with
325 : : * all concurrency issues! In practice it can't be stale enough to matter for
326 : : * the primary use case: index-only scans that check whether a heap fetch can
327 : : * be skipped.
328 : : *
329 : : * The argument for why it can't be stale enough to matter for the primary use
330 : : * case is as follows:
331 : : *
332 : : * Inserts: we need to detect that a VM bit was cleared by an insert right
333 : : * away, because the new tuple is present in the index but not yet visible.
334 : : * Reading the TID from the index page (under a shared lock on the index
335 : : * buffer) is serialized with the insertion of the TID into the index (under
336 : : * an exclusive lock on the same index buffer). Because the VM bit is cleared
337 : : * before the index is updated, and locking/unlocking of the index page acts
338 : : * as a full memory barrier, we are sure to see the cleared bit whenever we
339 : : * see a recently-inserted TID.
340 : : *
341 : : * Deletes: the clearing of the VM bit by a delete is NOT serialized with the
342 : : * index page access, because deletes do not update the index page (only
343 : : * VACUUM removes the index TID). So we may see a significantly stale value.
344 : : * However, we don't need to detect the delete right away, because the tuple
345 : : * remains visible until the deleting transaction commits or the statement
346 : : * ends (if it's our own transaction). In either case, the lock on the VM
347 : : * buffer will have been released (acting as a write barrier) after clearing
348 : : * the bit. And for us to have a snapshot that includes the deleting
349 : : * transaction (making the tuple invisible), we must have acquired
350 : : * ProcArrayLock after that time, acting as a read barrier.
351 : : */
352 : : uint8
353 : 5508316 : visibilitymap_get_status(Relation rel, BlockNumber heapBlk, Buffer *vmbuf)
354 : : {
355 : 5508316 : BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
356 : 5508316 : uint32 mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
357 : 5508316 : uint8 mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
358 : : char *map;
359 : : uint8 result;
360 : :
361 : : #ifdef TRACE_VISIBILITYMAP
362 : : elog(DEBUG1, "vm_get_status %s %d", RelationGetRelationName(rel), heapBlk);
363 : : #endif
364 : :
365 : : /* Reuse the old pinned buffer if possible */
366 [ + + ]: 5508316 : if (BufferIsValid(*vmbuf))
367 : : {
368 [ - + ]: 5350088 : if (BufferGetBlockNumber(*vmbuf) != mapBlock)
369 : : {
370 : 0 : ReleaseBuffer(*vmbuf);
371 : 0 : *vmbuf = InvalidBuffer;
372 : : }
373 : : }
374 : :
375 [ + + ]: 5508316 : if (!BufferIsValid(*vmbuf))
376 : : {
377 : 158228 : *vmbuf = vm_readbuf(rel, mapBlock, false);
378 [ + + ]: 158228 : if (!BufferIsValid(*vmbuf))
379 : 34261 : return (uint8) 0;
380 : : }
381 : :
382 : 5474055 : map = PageGetContents(BufferGetPage(*vmbuf));
383 : :
384 : : /*
385 : : * A single byte read is atomic. There could be memory-ordering effects
386 : : * here, but for performance reasons we make it the caller's job to worry
387 : : * about that.
388 : : */
389 : 5474055 : result = ((map[mapByte] >> mapOffset) & VISIBILITYMAP_VALID_BITS);
390 : 5474055 : return result;
391 : : }
392 : :
393 : : /*
394 : : * visibilitymap_count - count number of bits set in visibility map
395 : : *
396 : : * Note: we ignore the possibility of race conditions when the table is being
397 : : * extended concurrently with the call. New pages added to the table aren't
398 : : * going to be marked all-visible or all-frozen, so they won't affect the result.
399 : : */
400 : : void
401 : 138203 : visibilitymap_count(Relation rel, BlockNumber *all_visible, BlockNumber *all_frozen)
402 : : {
403 : : BlockNumber mapBlock;
404 : 138203 : BlockNumber nvisible = 0;
405 : 138203 : BlockNumber nfrozen = 0;
406 : :
407 : : /* all_visible must be specified */
408 : : Assert(all_visible);
409 : :
410 : 138203 : for (mapBlock = 0;; mapBlock++)
411 : 52588 : {
412 : : Buffer mapBuffer;
413 : : uint64 *map;
414 : :
415 : : /*
416 : : * Read till we fall off the end of the map. We assume that any extra
417 : : * bytes in the last page are zeroed, so we don't bother excluding
418 : : * them from the count.
419 : : */
420 : 190791 : mapBuffer = vm_readbuf(rel, mapBlock, false);
421 [ + + ]: 190791 : if (!BufferIsValid(mapBuffer))
422 : 138203 : break;
423 : :
424 : : /*
425 : : * We choose not to lock the page, since the result is going to be
426 : : * immediately stale anyway if anyone is concurrently setting or
427 : : * clearing bits, and we only really need an approximate value.
428 : : */
429 : 52588 : map = (uint64 *) PageGetContents(BufferGetPage(mapBuffer));
430 : :
431 : 52588 : nvisible += pg_popcount_masked((const char *) map, MAPSIZE, VISIBLE_MASK8);
432 [ + - ]: 52588 : if (all_frozen)
433 : 52588 : nfrozen += pg_popcount_masked((const char *) map, MAPSIZE, FROZEN_MASK8);
434 : :
435 : 52588 : ReleaseBuffer(mapBuffer);
436 : : }
437 : :
438 : 138203 : *all_visible = nvisible;
439 [ + - ]: 138203 : if (all_frozen)
440 : 138203 : *all_frozen = nfrozen;
441 : 138203 : }
442 : :
443 : : /*
444 : : * visibilitymap_prepare_truncate -
445 : : * prepare for truncation of the visibility map
446 : : *
447 : : * nheapblocks is the new size of the heap.
448 : : *
449 : : * Return the number of blocks of new visibility map.
450 : : * If it's InvalidBlockNumber, there is nothing to truncate;
451 : : * otherwise the caller is responsible for calling smgrtruncate()
452 : : * to truncate the visibility map pages.
453 : : */
454 : : BlockNumber
455 : 259 : visibilitymap_prepare_truncate(Relation rel, BlockNumber nheapblocks)
456 : : {
457 : : BlockNumber newnblocks;
458 : :
459 : : /* last remaining block, byte, and bit */
460 : 259 : BlockNumber truncBlock = HEAPBLK_TO_MAPBLOCK(nheapblocks);
461 : 259 : uint32 truncByte = HEAPBLK_TO_MAPBYTE(nheapblocks);
462 : 259 : uint8 truncOffset = HEAPBLK_TO_OFFSET(nheapblocks);
463 : :
464 : : #ifdef TRACE_VISIBILITYMAP
465 : : elog(DEBUG1, "vm_truncate %s %d", RelationGetRelationName(rel), nheapblocks);
466 : : #endif
467 : :
468 : : /*
469 : : * If no visibility map has been created yet for this relation, there's
470 : : * nothing to truncate.
471 : : */
472 [ - + ]: 259 : if (!smgrexists(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM))
473 : 0 : return InvalidBlockNumber;
474 : :
475 : : /*
476 : : * Unless the new size is exactly at a visibility map page boundary, the
477 : : * tail bits in the last remaining map page, representing truncated heap
478 : : * blocks, need to be cleared. This is not only tidy, but also necessary
479 : : * because we don't get a chance to clear the bits if the heap is extended
480 : : * again.
481 : : */
482 [ + + + + ]: 259 : if (truncByte != 0 || truncOffset != 0)
483 : 163 : {
484 : : Buffer mapBuffer;
485 : : Page page;
486 : : char *map;
487 : :
488 : 163 : newnblocks = truncBlock + 1;
489 : :
490 : 163 : mapBuffer = vm_readbuf(rel, truncBlock, false);
491 [ - + ]: 163 : if (!BufferIsValid(mapBuffer))
492 : : {
493 : : /* nothing to do, the file was already smaller */
494 : 0 : return InvalidBlockNumber;
495 : : }
496 : :
497 : 163 : page = BufferGetPage(mapBuffer);
498 : 163 : map = PageGetContents(page);
499 : :
500 : 163 : LockBuffer(mapBuffer, BUFFER_LOCK_EXCLUSIVE);
501 : :
502 : : /* NO EREPORT(ERROR) from here till changes are logged */
503 : 163 : START_CRIT_SECTION();
504 : :
505 : : /* Clear out the unwanted bytes. */
506 [ + + + - : 163 : MemSet(&map[truncByte + 1], 0, MAPSIZE - (truncByte + 1));
+ - - + -
- ]
507 : :
508 : : /*----
509 : : * Mask out the unwanted bits of the last remaining byte.
510 : : *
511 : : * ((1 << 0) - 1) = 00000000
512 : : * ((1 << 1) - 1) = 00000001
513 : : * ...
514 : : * ((1 << 6) - 1) = 00111111
515 : : * ((1 << 7) - 1) = 01111111
516 : : *----
517 : : */
518 : 163 : map[truncByte] &= (1 << truncOffset) - 1;
519 : :
520 : : /*
521 : : * Truncation of a relation is WAL-logged at a higher-level, and we
522 : : * will be called at WAL replay. But if checksums are enabled, we need
523 : : * to still write a WAL record to protect against a torn page, if the
524 : : * page is flushed to disk before the truncation WAL record. We cannot
525 : : * use MarkBufferDirtyHint here, because that will not dirty the page
526 : : * during recovery.
527 : : */
528 : 163 : MarkBufferDirty(mapBuffer);
529 [ + + + + : 163 : if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
+ + + - +
- + + +
- ]
530 : 142 : log_newpage_buffer(mapBuffer, false);
531 : :
532 : 163 : END_CRIT_SECTION();
533 : :
534 : 163 : UnlockReleaseBuffer(mapBuffer);
535 : : }
536 : : else
537 : 96 : newnblocks = truncBlock;
538 : :
539 [ + + ]: 259 : if (smgrnblocks(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM) <= newnblocks)
540 : : {
541 : : /* nothing to do, the file was already smaller than requested size */
542 : 163 : return InvalidBlockNumber;
543 : : }
544 : :
545 : 96 : return newnblocks;
546 : : }
547 : :
548 : : /*
549 : : * visibilitymap_truncation_length -
550 : : * compute truncation length for visibility map
551 : : *
552 : : * Given a proposed truncation length for the main fork, compute the
553 : : * correct truncation length for the visibility map. Should return the
554 : : * same answer as visibilitymap_prepare_truncate(), but without modifying
555 : : * anything.
556 : : */
557 : : BlockNumber
558 : 1 : visibilitymap_truncation_length(BlockNumber nheapblocks)
559 : : {
560 : 1 : return HEAPBLK_TO_MAPBLOCK_LIMIT(nheapblocks);
561 : : }
562 : :
563 : : /*
564 : : * Read a visibility map page.
565 : : *
566 : : * If the page doesn't exist, InvalidBuffer is returned, or if 'extend' is
567 : : * true, the visibility map file is extended.
568 : : */
569 : : static Buffer
570 : 892621 : vm_readbuf(Relation rel, BlockNumber blkno, bool extend)
571 : : {
572 : : Buffer buf;
573 : : SMgrRelation reln;
574 : :
575 : : /*
576 : : * Caution: re-using this smgr pointer could fail if the relcache entry
577 : : * gets closed. It's safe as long as we only do smgr-level operations
578 : : * between here and the last use of the pointer.
579 : : */
580 : 892621 : reln = RelationGetSmgr(rel);
581 : :
582 : : /*
583 : : * If we haven't cached the size of the visibility map fork yet, check it
584 : : * first.
585 : : */
586 [ + + ]: 892621 : if (reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber)
587 : : {
588 [ + + ]: 159181 : if (smgrexists(reln, VISIBILITYMAP_FORKNUM))
589 : 70590 : smgrnblocks(reln, VISIBILITYMAP_FORKNUM);
590 : : else
591 : 88591 : reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = 0;
592 : : }
593 : :
594 : : /*
595 : : * For reading we use ZERO_ON_ERROR mode, and initialize the page if
596 : : * necessary. It's always safe to clear bits, so it's better to clear
597 : : * corrupt pages than error out.
598 : : *
599 : : * We use the same path below to initialize pages when extending the
600 : : * relation, as a concurrent extension can end up with vm_extend()
601 : : * returning an already-initialized page.
602 : : */
603 [ + + ]: 892621 : if (blkno >= reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM])
604 : : {
605 [ + + ]: 177217 : if (extend)
606 : 4753 : buf = vm_extend(rel, blkno + 1);
607 : : else
608 : 172464 : return InvalidBuffer;
609 : : }
610 : : else
611 : 715404 : buf = ReadBufferExtended(rel, VISIBILITYMAP_FORKNUM, blkno,
612 : : RBM_ZERO_ON_ERROR, NULL);
613 : :
614 : : /*
615 : : * Initializing the page when needed is trickier than it looks, because of
616 : : * the possibility of multiple backends doing this concurrently, and our
617 : : * desire to not uselessly take the buffer lock in the normal path where
618 : : * the page is OK. We must take the lock to initialize the page, so
619 : : * recheck page newness after we have the lock, in case someone else
620 : : * already did it. Also, because we initially check PageIsNew with no
621 : : * lock, it's possible to fall through and return the buffer while someone
622 : : * else is still initializing the page (i.e., we might see pd_upper as set
623 : : * but other page header fields are still zeroes). This is harmless for
624 : : * callers that will take a buffer lock themselves, but some callers
625 : : * inspect the page without any lock at all. The latter is OK only so
626 : : * long as it doesn't depend on the page header having correct contents.
627 : : * Current usage is safe because PageGetContents() does not require that.
628 : : */
629 [ + + ]: 720157 : if (PageIsNew(BufferGetPage(buf)))
630 : : {
631 : 5097 : LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
632 [ + + ]: 5097 : if (PageIsNew(BufferGetPage(buf)))
633 : 5096 : PageInit(BufferGetPage(buf), BLCKSZ, 0);
634 : 5097 : LockBuffer(buf, BUFFER_LOCK_UNLOCK);
635 : : }
636 : 720157 : return buf;
637 : : }
638 : :
639 : : /*
640 : : * Ensure that the visibility map fork is at least vm_nblocks long, extending
641 : : * it if necessary with zeroed pages.
642 : : */
643 : : static Buffer
644 : 4753 : vm_extend(Relation rel, BlockNumber vm_nblocks)
645 : : {
646 : : Buffer buf;
647 : :
648 : 4753 : buf = ExtendBufferedRelTo(BMR_REL(rel), VISIBILITYMAP_FORKNUM, NULL,
649 : : EB_CREATE_FORK_IF_NEEDED |
650 : : EB_CLEAR_SIZE_CACHE,
651 : : vm_nblocks,
652 : : RBM_ZERO_ON_ERROR);
653 : :
654 : : /*
655 : : * Send a shared-inval message to force other backends to close any smgr
656 : : * references they may have for this rel, which we are about to change.
657 : : * This is a useful optimization because it means that backends don't have
658 : : * to keep checking for creation or extension of the file, which happens
659 : : * infrequently.
660 : : */
661 : 4753 : CacheInvalidateSmgr(RelationGetSmgr(rel)->smgr_rlocator);
662 : :
663 : 4753 : return buf;
664 : : }
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