linux-kernel - [PATCH] afs: Fix potential thrashing in afs writeback

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <164692725757.2097000.2060513769492301854.stgit@warthog.procyon.org.uk>
Date:   Thu, 10 Mar 2022 15:47:37 +0000
From:   David Howells <dhowells@...hat.com>
To:     marc.dionne@...istor.com
Cc:     linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
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 the 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@...istor.com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 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;
 		}