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

[Kairui Song]

On Tue, Mar 31, 2026 at 05:36:26PM +0800, Baolin Wang wrote:
> 
> 
> 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@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.
> 
> 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.

Ah, that's a very good point then. Can move it in V3, thanks!