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

[Baolin Wang]

On 3/26/26 1:04 PM, Barry Song wrote:
> 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>

Thanks.

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

Kairui is planning further optimizations here (including using a helper) 
[1], so it might be better to leave that for his series.

For this patch, I intend it to be a standalone fix, and I will add the 
Fixes: ac35a4902374 ("mm: multi-gen LRU: minimal implementation") tag.

[1] 
https://lore.kernel.org/all/20260318-mglru-reclaim-v1-7-2c46f9eb0508@xxxxxxxxxxx/

> > ---
> >   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