Re: [PATCH v2 12/12] mm/vmscan: unify writeback reclaim statistic and throttling

Baolin Wang
2026-03-31T09:24:50Z

On 3/29/26 3:52 AM, Kairui Song via B4 Relay 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. Also remove the
> folio_clear_reclaim in isolate_folio which was actively invalidating
> the congestion control. PG_reclaim is now handled by shrink_folio_list,
> keeping it in isolate_folio is not helpful.
>
> 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, congestion control is now effective and no more
> spin on LRU or premature OOM.
>
> Stress test on other workloads also looking good.
>
> Suggested-by: Chen Ridong <chenridong@huaweicloud.com>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
> mm/vmscan.c | 93 +++++++++++++++++++++++++++----------------------------------
> 1 file changed, 41 insertions(+), 52 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 1783da54ada1..83c8fdf8fdc4 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;
> @@ -4651,9 +4657,6 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
> if (!folio_test_referenced(folio))
> set_mask_bits(&folio->flags.f, LRU_REFS_MASK, 0);
>
> - /* for shrink_folio_list() */
> - folio_clear_reclaim(folio);

IMO, Moving this change into patch 8 would make more sense. Otherwise LGTM.