linux-kernel - [PATCH] netfs: Fix ceph copy to cache on write-begin

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Message-ID: <2117977.1733750054@warthog.procyon.org.uk>
Date: Mon, 09 Dec 2024 13:14:14 +0000
From: David Howells <dhowells@...hat.com>
To: Max Kellermann <max.kellermann@...os.com>
Cc: dhowells@...hat.com, Jeff Layton <jlayton@...nel.org>,
    netfs@...ts.linux.dev, linux-fsdevel <linux-fsdevel@...r.kernel.org>,
    linux-kernel@...r.kernel.org
Subject: [PATCH] netfs: Fix ceph copy to cache on write-begin

Hi Max,

Could you try this?

David
---
netfs: Fix ceph copy to cache on write-begin

At the end of netfs_unlock_read_folio() in which folios are marked
appropriately for copying to the cache (either with by being marked dirty
and having their private data set or by having PG_private_2 set) and then
unlocked, the folio_queue struct has the entry pointing to the folio
cleared.  This presents a problem for netfs_pgpriv2_write_to_the_cache(),
which is used to write folios marked with PG_private_2 to the cache as it
expects to be able to trawl the folio_queue list thereafter to find the
relevant folios, leading to a hang.

Fix this by not clearing the folio_queue entry if we're going to do the
deprecated copy-to-cache.  The clearance will be done instead as the folios
are written to the cache.

This can be reproduced by starting cachefiles, mounting a ceph filesystem
with "-o fsc" and writing to it.

Reported-by: Max Kellermann <max.kellermann@...os.com>
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Jeff Layton <jlayton@...nel.org>
cc: Ilya Dryomov <idryomov@...il.com>
cc: Xiubo Li <xiubli@...hat.com>
cc: netfs@...ts.linux.dev
cc: ceph-devel@...r.kernel.org
cc: linux-fsdevel@...r.kernel.org
---
 fs/netfs/read_collect.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/netfs/read_collect.c b/fs/netfs/read_collect.c
index 849f40f64443..72a16222b63b 100644
--- a/fs/netfs/read_collect.c
+++ b/fs/netfs/read_collect.c
@@ -62,10 +62,14 @@ static void netfs_unlock_read_folio(struct netfs_io_subrequest *subreq,
 		} else {
 			trace_netfs_folio(folio, netfs_folio_trace_read_done);
 		}
+
+		folioq_clear(folioq, slot);
 	} else {
 		// TODO: Use of PG_private_2 is deprecated.
 		if (test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags))
 			netfs_pgpriv2_mark_copy_to_cache(subreq, rreq, folioq, slot);
+		else
+			folioq_clear(folioq, slot);
 	}

 	if (!test_bit(NETFS_RREQ_DONT_UNLOCK_FOLIOS, &rreq->flags)) {
@@ -77,8 +81,6 @@ static void netfs_unlock_read_folio(struct netfs_io_subrequest *subreq,
 			folio_unlock(folio);
 		}
 	}
-
-	folioq_clear(folioq, slot);
 }

 /*