From 173ce1ca47c489135b2799f70f550e1319ba36d8 Mon Sep 17 00:00:00 2001
From: David Howells <dhowells@redhat.com>
Date: Fri, 11 Mar 2022 15:58:21 +0000
Subject: [PATCH] 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: 31143d5d515e ("AFS: implement basic file write support")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
Acked-by: Marc Dionne <marc.dionne@auristor.com>
Link: https://lore.kernel.org/r/164692725757.2097000.2060513769492301854.stgit@warthog.procyon.org.uk/ # v1
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
fs/afs/write.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/fs/afs/write.c b/fs/afs/write.c
index 5e9157d0da294..f447c902318da 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -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;
}
--
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