[PATCH] afs: Fix potential thrashing in afs writeback

[David Howells]

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 at auristor.com>
Signed-off-by: David Howells <dhowells at redhat.com>
Tested-by: Marc Dionne <marc.dionne at auristor.com>
Acked-by: Marc Dionne <marc.dionne at auristor.com>
Link: https://lore.kernel.org/r/164692725757.2097000.2060513769492301854.stgit@warthog.procyon.org.uk/ # v1
---

 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 85c9056ba9fb..bd0201f4939a 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -701,7 +701,7 @@ 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);
 
@@ -752,8 +752,15 @@ 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;
 		}