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Check access_count in lfc_evict (#9407)

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## Problem

See
https://neondb.slack.com/archives/C033A2WE6BZ/p1729007738526309?thread_ts=1722942856.987979&cid=C033A2WE6BZ

When replica receives WAL record which target page is not present in
shared buffer, we evict this page from LFC.
If all pages from the LFC chunk are evicted, then chunk is moved to the
beginning of LRU least to force it reuse.
Unfortunately access_count is not checked and if the entry is access at
this moment then this operation can cause LRU list corruption.

## Summary of changes

Check `access_count` in `lfc_evict`

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
This commit is contained in:
Konstantin Knizhnik
2024-10-17 08:04:57 +03:00
committed by GitHub
parent 67d5d98b19
commit 934dbb61f5
1 changed files with 24 additions and 21 deletions
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45
pgxn/neon/file_cache.c
View File

@@ -617,31 +617,34 @@ lfc_evict(NRelFileInfo rinfo, ForkNumber forkNum, BlockNumber blkno)
/* remove the page from the cache */
entry->bitmap[chunk_offs >> 5] &= ~(1 << (chunk_offs & (32 - 1)));
/*
* If the chunk has no live entries, we can position the chunk to be
* recycled first.
*/
if (entry->bitmap[chunk_offs >> 5] == 0)
if (entry->access_count == 0)
{
bool has_remaining_pages = false;
for (int i = 0; i < CHUNK_BITMAP_SIZE; i++)
{
if (entry->bitmap[i] != 0)
{
has_remaining_pages = true;
break;
}
}
/*
* Put the entry at the position that is first to be reclaimed when we
* have no cached pages remaining in the chunk
* If the chunk has no live entries, we can position the chunk to be
* recycled first.
*/
if (!has_remaining_pages)
if (entry->bitmap[chunk_offs >> 5] == 0)
{
dlist_delete(&entry->list_node);
dlist_push_head(&lfc_ctl->lru, &entry->list_node);
bool has_remaining_pages = false;
for (int i = 0; i < CHUNK_BITMAP_SIZE; i++)
{
if (entry->bitmap[i] != 0)
{
has_remaining_pages = true;
break;
}
}
/*
* Put the entry at the position that is first to be reclaimed when we
* have no cached pages remaining in the chunk
*/
if (!has_remaining_pages)
{
dlist_delete(&entry->list_node);
dlist_push_head(&lfc_ctl->lru, &entry->list_node);
}
}
}