[PATCH 1/2] jbd2: check need_resched() when skipping busy checkpoint buffers

[Max Kellermann <max.kellermann@ionos.com>]

journal_shrink_one_cp_list() skips busy checkpoint buffers when called
with JBD2_SHRINK_BUSY_SKIP.  The continue statement on this path also
skips the need_resched() check at the end of the loop body.

Consequently, when a checkpoint list contains mostly busy buffers, the
shrinker can walk the entire list while holding journal->j_list_lock,
even when a reschedule has been requested.  Large checkpoint lists under
memory pressure can therefore cause long lock hold times and leave other
CPUs spinning on j_list_lock, resulting in soft lockups or RCU stalls.

Route the busy-buffer path through the need_resched() check so that the
shrinker can release j_list_lock and reschedule promptly, restoring
parity with the clean-buffer path, which already checks need_resched().
This does not change which checkpoint buffers are eligible for removal.

Fixes: b98dba273a0e ("jbd2: remove journal_clean_one_cp_list()")
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 fs/jbd2/checkpoint.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index 1508e2f54462..5266017565ac 100644
--- a/fs/jbd2/checkpoint.c
+++ b/fs/jbd2/checkpoint.c
@@ -389,7 +389,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
 			ret = jbd2_journal_try_remove_checkpoint(jh);
 			if (ret < 0) {
 				if (type == JBD2_SHRINK_BUSY_SKIP)
-					continue;
+					goto next;
 				break;
 			}
 		}
@@ -400,6 +400,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
 			break;
 		}
 
+next:
 		if (need_resched())
 			break;
 	} while (jh != last_jh);
-- 
2.47.3