Re: [RFC PATCH] mm: vmscan: fix dirty folios throttling on cgroup v1 for MGLRU

[Barry Song]

On Wed, Mar 25, 2026 at 7:51 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> 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>

LGTM,

Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

Maybe we can extract a common inline helper to avoid the copy-paste duplication.

> ---
>  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;
>  }
> --
> 2.47.3
>