Commit 173ce1ca authored by David Howells's avatar David Howells Committed by Linus Torvalds
Browse files

afs: Fix potential thrashing in afs writeback 



In afs_writepages_region(), if the dirty page we find is undergoing
writeback or write to cache, but the sync_mode is WB_SYNC_NONE, we go
round the loop trying the same page again and again with no pausing or
waiting unless and until another thread manages to clear the writeback
and fscache flags.

Fix this with three measures:

 (1) Advance start to after the page we found.

 (2) Break out of the loop and return if rescheduling is requested.

 (3) Arbitrarily give up after a maximum of 5 skips.

Fixes: 31143d5d ("AFS: implement basic file write support")
Reported-by: default avatarMarc Dionne <marc.dionne@auristor.com>
Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Tested-by: default avatarMarc Dionne <marc.dionne@auristor.com>
Acked-by: default avatarMarc Dionne <marc.dionne@auristor.com>
Link: https://lore.kernel.org/r/164692725757.2097000.2060513769492301854.stgit@warthog.procyon.org.uk/

 # v1
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4edc0760

fs/afs/write.c

+8 −1
Original line number Diff line number Diff line
@@ -703,7 +703,7 @@ static int afs_writepages_region(struct address_space *mapping, 
	struct folio *folio;

	struct page *head_page;

	ssize_t ret;

	int n;

	int n, skips = 0;



	_enter("%llx,%llx,", start, end);
@@ -754,8 +754,15 @@ static int afs_writepages_region(struct address_space *mapping, 
#ifdef CONFIG_AFS_FSCACHE

				folio_wait_fscache(folio);

#endif

			} else {

				start += folio_size(folio);

			}

			folio_put(folio);

			if (wbc->sync_mode == WB_SYNC_NONE) {

				if (skips >= 5 || need_resched())

					break;

				skips++;

			}

			continue;

		}