Re: [RFC PATCH] mm: vmscan: fix dirty folios throttling on cgroup v1 for MGLRU

[Baolin Wang <baolin.wang@linux.alibaba.com>]

CC Lorenzo.

(Sorry, Lorenzo. I switched to use your new email address, but forgot to 
CC you.)

On 3/25/26 7:50 PM, Baolin Wang wrote:
> The balance_dirty_pages() won't do the dirty folios throttling on cgroupv1.
> See commit 9badce000e2c ("cgroup, writeback: don't enable cgroup writeback
> on traditional hierarchies").
> 
> Moreover, after commit 6b0dfabb3555 ("fs: Remove aops->writepage"), we no
> longer attempt to write back filesystem folios through reclaim.
> 
> On large memory systems, the flusher may not be able to write back quickly
> enough. Consequently, MGLRU will encounter many folios that are already
> under writeback. Since we cannot reclaim these dirty folios, the system
> may run out of memory and trigger the OOM killer.
> 
> Hence, for cgroup v1, let's throttle reclaim after waking up the flusher,
> which is similar to commit 81a70c21d917 ("mm/cgroup/reclaim: fix dirty
> pages throttling on cgroup v1"), to avoid unnecessary OOM.
> 
> The following test program can easily reproduce the OOM issue. With this patch
> applied, the test passes successfully.
> 
> $mkdir /sys/fs/cgroup/memory/test
> $echo 256M > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> $echo $$ > /sys/fs/cgroup/memory/test/cgroup.procs
> $dd if=/dev/zero of=/mnt/data.bin bs=1M count=800
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>   mm/vmscan.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 33287ba4a500..a9648269fae8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -5036,9 +5036,20 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>   	 * If too many file cache in the coldest generation can't be evicted
>   	 * due to being dirty, wake up the flusher.
>   	 */
> -	if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken)
> +	if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken) {
> +		struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +
>   		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);
> +	}
> +
>   	/* whether this lruvec should be rotated */
>   	return nr_to_scan < 0;
>   }