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

[Kairui Song]

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@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.

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.