mm: vmscan: fix dirty folios throttling on cgroup v1 for MGLRU

The balance_dirty_pages() won't do the dirty folios throttling on cgroupv1.
See commit 9badce0 ("cgroup, writeback: don't enable cgroup writeback
on traditional hierarchies").

Moreover, after commit 6b0dfab ("fs: Remove aops->writepage"), we no
longer attempt to write back filesystem folios through reclaim.

On large memory systems, the flusher may not be able to write back quickly
enough. Consequently, MGLRU will encounter many folios that are already
under writeback. Since we cannot reclaim these dirty folios, the system
may run out of memory and trigger the OOM killer.

Hence, for cgroup v1, let's throttle reclaim after waking up the flusher,
which is similar to commit 81a70c2 ("mm/cgroup/reclaim: fix dirty
pages throttling on cgroup v1"), to avoid unnecessary OOM.

The following test program can easily reproduce the OOM issue. With this patch
applied, the test passes successfully.

$mkdir /sys/fs/cgroup/memory/test
$echo 256M > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
$echo $$ > /sys/fs/cgroup/memory/test/cgroup.procs
$dd if=/dev/zero of=/mnt/data.bin bs=1M count=800

Fixes: ac35a49 ("mm: multi-gen LRU: minimal implementation")
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Author: Baolin Wang

mm/vmscan.c

@@ -4896,9 +4896,24 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 	 * If too many file cache in the coldest generation can't be evicted
 	 * due to being dirty, wake up the flusher.
 	 */
-	if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken)
+	if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken) {
+		struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+
 		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);
+	}
+
 	/* whether this lruvec should be rotated */
 	return nr_to_scan < 0;
 }