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

[Leno Hou <lenohou@gmail.com>]

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
> 

Hi Kairui,

I have tested this patch series on both stable v6.1.163 and 7.0.0-rc5
regarding the writeback throttling issue you described on arm64 platform.


Test Results:
1. Kernel 6.1.163: I was unable to reproduce the OOM issue with the 
provided test script. This is expected as the MGLRU writeback handling 
might behave differently or the issue is less prominent in this specific 
stable branch.

2. Kernel 7.0.0-rc5: I successfully reproduced the OOM issue using your 
bash script. The dd process gets OOM killed shortly after starting the 
writeback pressure.

Verification:
After applying your patch, I re-ran the test on 7.0-rc5:
1. The congestion control/throttling is now working as expected.
2. The OOM issue is resolved, and the dd process completes successfully 
without being killed.


Tested-by: Leno Hou <lenohou@gmail.com>

> 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);
> -
>   	success = lru_gen_del_folio(lruvec, folio, true);
>   	VM_WARN_ON_ONCE_FOLIO(!success, folio);
>   
> @@ -4833,26 +4836,11 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   retry:
>   	reclaimed = shrink_folio_list(&list, pgdat, sc, &stat, false, memcg);
>   	sc->nr_reclaimed += reclaimed;
> +	handle_reclaim_writeback(isolated, pgdat, sc, &stat);
>   	trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
>   			type_scanned, reclaimed, &stat, sc->priority,
>   			type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>   
> -	/*
> -	 * If too many file cache in the coldest generation can't be evicted
> -	 * due to being dirty, wake up the flusher.
> -	 */
> -	if (stat.nr_unqueued_dirty == isolated) {
> -		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()).
> -		 */
> -		if (!writeback_throttling_sane(sc))
> -			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
> -	}
> -
>   	list_for_each_entry_safe_reverse(folio, next, &list, lru) {
>   		DEFINE_MIN_SEQ(lruvec);
>   
> @@ -4895,6 +4883,7 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   
>   	if (!list_empty(&list)) {
>   		skip_retry = true;
> +		isolated = 0;
>   		goto retry;
>   	}
>   
> 


--
Best regards,
Leno Hou