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

Baolin Wang
2026-03-31T09:36:35Z

On 3/31/26 5:29 PM, Kairui Song wrote:
> 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.

I’m not referring to all the above changes. What I mean is that the
'folio_clear_reclaim' removal should belong to patch 8. Since
shrink_folio_list() in patch 8 will handle the writeback logic,
folio_clear_reclaim() should also be removed in the same patch.