Re: [RFC PATCH] mm: vmscan: fix dirty folios throttling on cgroup v1 for MGLRU
From: Baolin Wang
Date: Wed Mar 25 2026 - 22:28:13 EST
On 3/25/26 9:35 PM, Kairui Song wrote:
On Wed, Mar 25, 2026 at 09:20:55PM +0800, Baolin Wang wrote:
Hi Kairui,
On 3/25/26 8:07 PM, Kairui Song wrote:
On Wed, Mar 25, 2026 at 07:50:40PM +0800, Baolin Wang wrote:
The balance_dirty_pages() won't do the dirty folios throttling on cgroupv1.
See commit 9badce000e2c ("cgroup, writeback: don't enable cgroup writeback
on traditional hierarchies").
Moreover, after commit 6b0dfabb3555 ("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 81a70c21d917 ("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
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 33287ba4a500..a9648269fae8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5036,9 +5036,20 @@ 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()).
+ */
+ if (!writeback_throttling_sane(sc))
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
+ }
+
/* whether this lruvec should be rotated */
return nr_to_scan < 0;
}
Hi Baolin
Interesting I want to fix this too, after or with:
https://lore.kernel.org/linux-mm/20260318-mglru-reclaim-v1-0-2c46f9eb0508@xxxxxxxxxxx/
Thanks for taking a look.
With current fix you posted, MGLRU's dirty throttling is still
a bit different from active / inactive LRU. In fact MGLRU
treat dirty folios quite differently causing many other issues too,
e.g. it's much more likely for dirty folios to stuck at the tail
for MGLRU so simply apply the throttling could cause too
aggressive throttling. Or batch is too large to trigger the
throttling.
Thanks for sharing this.
Hi Baolin,
So I'm planning to add below patch to V2 of that series (also this
is suggested by Ridong), how do you think? There are several
other throttling things to be fixed too, more than just the
V1 support. I can have your suggested-by too.
But I still think this fix deserves its own commit, because this is indeed
fixing a real issue that I ran into. Even if the throttling isn't perfect
for cgroup v1, it aligns with the legacy-LRU behavior and is essential to
avoid premature OOMs firstly. MGLRU dirty folio handling improvement can be
done as a separate optimization in your series.
Anyway, let's also wait for more feedback from others.
Sure, fixing this first is fine to me, just saying that you may
still see unexpected throttling or ineffective throttling with this.
This is no conflict between these two approach. I can rebase that
series on top of yours, and that series would help to solve the
rest of issues.
OK. Thanks.