[PATCH v2] ceph: fix writeback_count leak in write_folio_nounlock()

[Wentao Liang <vulab@iscas.ac.cn>]

write_folio_nounlock() increments fsc->writeback_count to track
in-flight writeback operations. On several error paths where the
function returns early (folio lookup failure, snapshot context
allocation failure, and writepages submission failure), the function
returns without calling atomic_long_dec() to decrement the counter.

Each leaked increment keeps the counter above zero, which can prevent
the filesystem from cleanly unmounting or suspending writes.

Add atomic_long_dec() calls on all error paths that currently return
without decrementing the counter.

Fixes: d55207717ded ("ceph: add encryption support to writepage and writepages")
Cc: stable@vger.kernel.org
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>

---
Change in v2:
- Also clear write_congested flag when decrementing writeback_count
  on error paths, as suggested by Viacheslav Dubeyko.
---
 fs/ceph/addr.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index a606378649c3..7fab73874068 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -790,7 +790,9 @@ static int write_folio_nounlock(struct folio *folio,
 				    ceph_wbc.truncate_size, true);
 	if (IS_ERR(req)) {
 		folio_redirty_for_writepage(wbc, folio);
-		atomic_long_dec(&fsc->writeback_count);
+		if (atomic_long_dec_return(&fsc->writeback_count) <
+				CONGESTION_OFF_THRESH(fsc->mount_options->congestion_kb))
+			fsc->write_congested = false;
 		return PTR_ERR(req);
 	}
 
@@ -810,7 +812,9 @@ static int write_folio_nounlock(struct folio *folio,
 			folio_redirty_for_writepage(wbc, folio);
 			folio_end_writeback(folio);
 			ceph_osdc_put_request(req);
-			atomic_long_dec(&fsc->writeback_count);
+			if (atomic_long_dec_return(&fsc->writeback_count) <
+					CONGESTION_OFF_THRESH(fsc->mount_options->congestion_kb))
+				fsc->write_congested = false;
 			return PTR_ERR(bounce_page);
 		}
 	}
@@ -849,7 +853,9 @@ static int write_folio_nounlock(struct folio *folio,
 			      ceph_vinop(inode), folio);
 			folio_redirty_for_writepage(wbc, folio);
 			folio_end_writeback(folio);
-			atomic_long_dec_return(&fsc->writeback_count);
+			if (atomic_long_dec_return(&fsc->writeback_count) <
+					CONGESTION_OFF_THRESH(fsc->mount_options->congestion_kb))
+				fsc->write_congested = false;
 			return err;
 		}
 		if (err == -EBLOCKLISTED)
-- 
2.34.1