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

[Baolin Wang]

On 3/29/26 3:52 AM, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>   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.