[PATCH] mm/cgroup/reclaim: Fix dirty pages throttling on cgroup v1

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

balance_dirty_pages doesn't do the required dirty throttling on cgroupv1. See
commit 9badce000e2c ("cgroup, writeback: don't enable cgroup writeback on
traditional hierarchies"). Instead, the kernel depends on writeback throttling
in shrink_folio_list to achieve the same goal. With large memory systems, the
flusher may not be able to writeback quickly enough such that we will start
finding pages in the shrink_folio_list already in writeback. Hence for cgroupv1
let's do a reclaim throttle after waking up the flusher.

The below test which used to fail on a 256GB system completes till the
the file system is full with this change.

root@lp2:/sys/fs/cgroup/memory# mkdir test
root@lp2:/sys/fs/cgroup/memory# cd test/
root@lp2:/sys/fs/cgroup/memory/test# echo 120M > memory.limit_in_bytes
root@lp2:/sys/fs/cgroup/memory/test# echo $$ > tasks
root@lp2:/sys/fs/cgroup/memory/test# dd if=/dev/zero of=/home/kvaneesh/test bs=1M
Killed

Cc: <stable@kernel.org>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
---
 mm/vmscan.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 04d8b88e5216..388022c5ef2b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2514,8 +2514,20 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 	 * the flushers simply cannot keep up with the allocation
 	 * rate. Nudge the flusher threads in case they are asleep.
 	 */
-	if (stat.nr_unqueued_dirty == nr_taken)
+	if (stat.nr_unqueued_dirty == nr_taken) {
 		wakeup_flusher_threads(WB_REASON_VMSCAN);
+		/*
+		 * For cgroupv1 dirty throttling is achieved by waking up
+		 * the kernel flusher here and later waiting on folios
+		 * which are in writeback to finish (see shrink_folio_list()).
+		 *
+		 * Flusher may not be able to issue writeback quickly
+		 * enough for cgroupv1 writeback throttling to work
+		 * on a large system.
+		 */
+		if (!writeback_throttling_sane(sc))
+			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
+	}
 
 	sc->nr.dirty += stat.nr_dirty;
 	sc->nr.congested += stat.nr_congested;
-- 
2.38.1