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

Kairui Song
2026-03-31T09:30:14Z

On Tue, Mar 31, 2026 at 05:24:39PM +0800, Baolin Wang wrote:
>
>
> 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.

Thanks for the review! I made it a separate patch so we can better
identify which part had the performance gain, and patch 8 can keep
the review by. Patch 8 is still good without this, a few counters
are updated with no user, kind of wasted but that's harmless.