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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Sun, Mar 29, 2026 at 03:52:38AM +0800, 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

What do you mean by congestion control here?

> 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>

There is still differences for global and kswapd reclaim in the shrink_node()
like kswapd throttling and congestion state management and throttling. Any plan
to unify them?