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Commitc780a7a

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Add CheckBuffer() to check on-disk pages without shared buffer loading
CheckBuffer() is designed to be a concurrent-safe function able to runsanity checks on a relation page without loading it into the sharedbuffers. The operation is done using a lock on the partition involvedin the shared buffer mapping hashtable and an I/O lock for the bufferitself, preventing the risk of false positives due to any concurrentactivity.The primary use of this function is the detection of on-disk corruptionsfor relation pages. If a page is found in shared buffers, the on-diskpage is checked if not dirty (a follow-up checkpoint would flush a validversion of the page if dirty anyway), as it could be possible that apage was present for a long time in shared buffers with its on-diskversion corrupted. Such a scenario could lead to a corrupted cluster ifa host is plugged off for example. If the page is not found in sharedbuffers, its on-disk state is checked. PageIsVerifiedExtended() is usedto apply the same sanity checks as when a page gets loaded into sharedbuffers.This function will be used by an upcoming patch able to check the stateof on-disk relation pages using a SQL function.Author: Julien Rouhaud, Michael PaquierReviewed-by: Masahiko SawadaDiscussion:https://postgr.es/m/CAOBaU_aVvMjQn=ge5qPiJOPMmOj5=ii3st5Q0Y+WuLML5sR17w@mail.gmail.com
1 parent9e0f87a commitc780a7a

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2 files changed

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2 files changed

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‎src/backend/storage/buffer/bufmgr.c

Lines changed: 92 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -4585,3 +4585,95 @@ TestForOldSnapshot_impl(Snapshot snapshot, Relation relation)
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(errcode(ERRCODE_SNAPSHOT_TOO_OLD),
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errmsg("snapshot too old")));
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}
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/*
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* CheckBuffer
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*
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* Check the state of a buffer without loading it into the shared buffers. To
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* avoid torn pages and possible false positives when reading data, a shared
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* LWLock is taken on the target buffer pool partition mapping, and we check
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* if the page is in shared buffers or not. An I/O lock is taken on the block
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* to prevent any concurrent activity from happening.
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*
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* If the page is found as dirty in the shared buffers, it is ignored as
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* it will be flushed to disk either before the end of the next checkpoint
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* or during recovery in the event of an unsafe shutdown.
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*
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* If the page is found in the shared buffers but is not dirty, we still
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* check the state of its data on disk, as it could be possible that the
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* page stayed in shared buffers for a rather long time while the on-disk
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* data got corrupted.
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*
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* If the page is not found in shared buffers, the block is read from disk
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* while holding the buffer pool partition mapping LWLock.
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*
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* The page data is stored in a private memory area local to this function
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* while running the checks.
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*/
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bool
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CheckBuffer(SMgrRelationsmgr,ForkNumberforknum,BlockNumberblkno)
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{
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charbuffer[BLCKSZ];
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BufferTagbuf_tag;/* identity of requested block */
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uint32buf_hash;/* hash value for buf_tag */
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LWLock*partLock;/* buffer partition lock for the buffer */
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BufferDesc*bufdesc;
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intbuf_id;
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Assert(smgrexists(smgr,forknum));
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/* create a tag so we can look after the buffer */
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INIT_BUFFERTAG(buf_tag,smgr->smgr_rnode.node,forknum,blkno);
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/* determine its hash code and partition lock ID */
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buf_hash=BufTableHashCode(&buf_tag);
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partLock=BufMappingPartitionLock(buf_hash);
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/* see if the block is in the buffer pool or not */
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LWLockAcquire(partLock,LW_SHARED);
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buf_id=BufTableLookup(&buf_tag,buf_hash);
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if (buf_id >=0)
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{
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uint32buf_state;
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/*
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* Found it. Now, retrieve its state to know what to do with it, and
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* release the pin immediately. We do so to limit overhead as much as
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* possible. We keep the shared LWLock on the target buffer mapping
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* partition for now, so this buffer cannot be evicted, and we acquire
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* an I/O Lock on the buffer as we may need to read its contents from
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* disk.
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*/
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bufdesc=GetBufferDescriptor(buf_id);
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LWLockAcquire(BufferDescriptorGetIOLock(bufdesc),LW_SHARED);
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buf_state=LockBufHdr(bufdesc);
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UnlockBufHdr(bufdesc,buf_state);
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/* If the page is dirty or invalid, skip it */
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if ((buf_state&BM_DIRTY)!=0|| (buf_state&BM_TAG_VALID)==0)
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{
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LWLockRelease(BufferDescriptorGetIOLock(bufdesc));
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LWLockRelease(partLock);
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return true;
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}
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/* Read the buffer from disk, with the I/O lock still held */
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smgrread(smgr,forknum,blkno,buffer);
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LWLockRelease(BufferDescriptorGetIOLock(bufdesc));
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}
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else
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{
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/*
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* Simply read the buffer. There's no risk of modification on it as
4670+
* we are holding the buffer pool partition mapping lock.
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*/
4672+
smgrread(smgr,forknum,blkno,buffer);
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}
4674+
4675+
/* buffer lookup done, so now do its check */
4676+
LWLockRelease(partLock);
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4678+
returnPageIsVerifiedExtended(buffer,blkno,PIV_REPORT_STAT);
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}

‎src/include/storage/bufmgr.h

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -240,6 +240,9 @@ extern void AtProcExit_LocalBuffers(void);
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externvoidTestForOldSnapshot_impl(Snapshotsnapshot,Relationrelation);
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243+
externboolCheckBuffer(structSMgrRelationData*smgr,ForkNumberforknum,
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BlockNumberblkno);
245+
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/* in freelist.c */
244247
externBufferAccessStrategyGetAccessStrategy(BufferAccessStrategyTypebtype);
245248
externvoidFreeAccessStrategy(BufferAccessStrategystrategy);

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