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

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

On 3/25/26 9:35 PM, Kairui Song wrote:
> On Wed, Mar 25, 2026 at 09:20:55PM +0800, Baolin Wang wrote:
>> Hi Kairui,
>>
>> On 3/25/26 8:07 PM, Kairui Song wrote:
>>> On Wed, Mar 25, 2026 at 07:50:40PM +0800, 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;
>>>>    }
>>>
>>> Hi Baolin
>>>
>>> Interesting I want to fix this too, after or with:
>>> https://lore.kernel.org/linux-mm/20260318-mglru-reclaim-v1-0-2c46f9eb0508@tencent.com/
>>
>> Thanks for taking a look.
>>
>>>
>>> With current fix you posted, MGLRU's dirty throttling is still
>>> a bit different from active / inactive LRU. In fact MGLRU
>>> treat dirty folios quite differently causing many other issues too,
>>> e.g. it's much more likely for dirty folios to stuck at the tail
>>> for MGLRU so simply apply the throttling could cause too
>>> aggressive throttling. Or batch is too large to trigger the
>>> throttling.
>>
>> Thanks for sharing this.
> 
> Hi Baolin,
> 
>>
>>> So I'm planning to add below patch to V2 of that series (also this
>>> is suggested by Ridong), how do you think? There are several
>>> other throttling things to be fixed too, more than just the
>>> V1 support. I can have your suggested-by too.
>>
>> But I still think this fix deserves its own commit, because this is indeed
>> fixing a real issue that I ran into. Even if the throttling isn't perfect
>> for cgroup v1, it aligns with the legacy-LRU behavior and is essential to
>> avoid premature OOMs firstly. MGLRU dirty folio handling improvement can be
>> done as a separate optimization in your series.
>>
>> Anyway, let's also wait for more feedback from others.
>>
> 
> Sure, fixing this first is fine to me, just saying that you may
> still see unexpected throttling or ineffective throttling with this.
> 
> This is no conflict between these two approach. I can rebase that
> series on top of yours, and that series would help to solve the
> rest of issues.

OK. Thanks.