Re: [PATCH v3 14/14] mm/vmscan: unify writeback reclaim statistic and throttling

[Axel Rasmussen <axelrasmussen@google.com>]

On Thu, Apr 2, 2026 at 11:53 AM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@kernel.org> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> Currently MGLRU and non-MGLRU handle the reclaim statistic and
> writeback handling very differently, especially throttling.
> Basically MGLRU just ignored the throttling part.
>
> Let's just unify this part, use a helper to deduplicate the code
> so both setups will share the same behavior.
>
> Test using following reproducer using bash:
>
>   echo "Setup a slow device using dm delay"
>   dd if=/dev/zero of=/var/tmp/backing bs=1M count=2048
>   LOOP=$(losetup --show -f /var/tmp/backing)
>   mkfs.ext4 -q $LOOP
>   echo "0 $(blockdev --getsz $LOOP) delay $LOOP 0 0 $LOOP 0 1000" | \
>       dmsetup create slow_dev
>   mkdir -p /mnt/slow && mount /dev/mapper/slow_dev /mnt/slow
>
>   echo "Start writeback pressure"
>   sync && echo 3 > /proc/sys/vm/drop_caches
>   mkdir /sys/fs/cgroup/test_wb
>   echo 128M > /sys/fs/cgroup/test_wb/memory.max
>   (echo $BASHPID > /sys/fs/cgroup/test_wb/cgroup.procs && \
>       dd if=/dev/zero of=/mnt/slow/testfile bs=1M count=192)
>
>   echo "Clean up"
>   echo "0 $(blockdev --getsz $LOOP) error" | dmsetup load slow_dev
>   dmsetup resume slow_dev
>   umount -l /mnt/slow && sync
>   dmsetup remove slow_dev
>
> Before this commit, `dd` will get OOM killed immediately if
> MGLRU is enabled. Classic LRU is fine.
>
> After this commit, throttling is now effective and no more spin on
> LRU or premature OOM. Stress test on other workloads also looking good.
>
> Global throttling is not here yet, we will fix that separately later.

If I understand correctly, I think this fixes this regression report
[1] from a long time ago that was never fully resolved?

[1]: https://lore.kernel.org/lkml/ZeC-u7GRSptoVqia@chrisdown.name/

We investigated at that time, but I don't feel we got to a consensus
on how to solve it. I think we got a bit bogged down trying to
"completely solve writeback throttling" rather than just doing some
incremental improvement which fixed that particular case.

>
> Suggested-by: Chen Ridong <chenridong@huaweicloud.com>
> Tested-by: Leno Hou <lenohou@gmail.com>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/vmscan.c | 90 ++++++++++++++++++++++++++++---------------------------------
>  1 file changed, 41 insertions(+), 49 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9120d914445e..a7b3e5b6676b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1942,6 +1942,44 @@ static int current_may_throttle(void)
>         return !(current->flags & PF_LOCAL_THROTTLE);
>  }
>
> +static void handle_reclaim_writeback(unsigned long nr_taken,
> +                                    struct pglist_data *pgdat,
> +                                    struct scan_control *sc,
> +                                    struct reclaim_stat *stat)
> +{
> +       /*
> +        * If dirty folios are scanned that are not queued for IO, it
> +        * implies that flushers are not doing their job. This can
> +        * happen when memory pressure pushes dirty folios to the end of
> +        * the LRU before the dirty limits are breached and the dirty
> +        * data has expired. It can also happen when the proportion of
> +        * dirty folios grows not through writes but through memory
> +        * pressure reclaiming all the clean cache. And in some cases,
> +        * 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 && 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;
> +       sc->nr.writeback += stat->nr_writeback;
> +       sc->nr.immediate += stat->nr_immediate;
> +       sc->nr.taken += nr_taken;
> +}
> +
>  /*
>   * shrink_inactive_list() is a helper for shrink_node().  It returns the number
>   * of reclaimed pages
> @@ -2005,39 +2043,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>         lruvec_lock_irq(lruvec);
>         lru_note_cost_unlock_irq(lruvec, file, stat.nr_pageout,
>                                         nr_scanned - nr_reclaimed);
> -
> -       /*
> -        * If dirty folios are scanned that are not queued for IO, it
> -        * implies that flushers are not doing their job. This can
> -        * happen when memory pressure pushes dirty folios to the end of
> -        * the LRU before the dirty limits are breached and the dirty
> -        * data has expired. It can also happen when the proportion of
> -        * dirty folios grows not through writes but through memory
> -        * pressure reclaiming all the clean cache. And in some cases,
> -        * 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) {
> -               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;
> -       sc->nr.writeback += stat.nr_writeback;
> -       sc->nr.immediate += stat.nr_immediate;
> -       sc->nr.taken += nr_taken;
> -
> +       handle_reclaim_writeback(nr_taken, pgdat, sc, &stat);
>         trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
>                         nr_scanned, nr_reclaimed, &stat, sc->priority, file);
>         return nr_reclaimed;
> @@ -4824,26 +4830,11 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  retry:
>         reclaimed = shrink_folio_list(&list, pgdat, sc, &stat, false, memcg);
>         sc->nr_reclaimed += reclaimed;
> +       handle_reclaim_writeback(isolated, pgdat, sc, &stat);
>         trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
>                         type_scanned, reclaimed, &stat, sc->priority,
>                         type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>
> -       /*
> -        * If too many file cache in the coldest generation can't be evicted
> -        * due to being dirty, wake up the flusher.
> -        */
> -       if (stat.nr_unqueued_dirty == isolated) {
> -               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);
> -       }
> -
>         list_for_each_entry_safe_reverse(folio, next, &list, lru) {
>                 DEFINE_MIN_SEQ(lruvec);
>
> @@ -4886,6 +4877,7 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>
>         if (!list_empty(&list)) {
>                 skip_retry = true;
> +               isolated = 0;
>                 goto retry;
>         }
>
>
> --
> 2.53.0
>
>