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

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

On 3/26/26 1:04 PM, Barry Song wrote:
> On Wed, Mar 25, 2026 at 7:51 PM Baolin Wang
> <baolin.wang@linux.alibaba.com> 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>
> 
> LGTM,
> 
> Reviewed-by: Barry Song <baohua@kernel.org>

Thanks.

> Maybe we can extract a common inline helper to avoid the copy-paste duplication.

Kairui is planning further optimizations here (including using a helper) 
[1], so it might be better to leave that for his series.

For this patch, I intend it to be a standalone fix, and I will add the 
Fixes: ac35a4902374 ("mm: multi-gen LRU: minimal implementation") tag.

[1] 
https://lore.kernel.org/all/20260318-mglru-reclaim-v1-7-2c46f9eb0508@tencent.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;
>>   }
>> --
>> 2.47.3
>>