Re: [PATCH v3] mm: mglru: fix stale batch updates after memcg reparenting

[Qi Zheng <qi.zheng@linux.dev>]

Hi Shakeel,

On 6/26/26 4:22 AM, Shakeel Butt wrote:
> On Thu, Jun 25, 2026 at 11:15:54PM +0800, Qi Zheng wrote:
>> From: Qi Zheng <zhengqi.arch@bytedance.com>
>>
>> The mglru page table walker batches per-generation size deltas in
>> walk->nr_pages while walking page tables without holding the lruvec lock.
>> The reset_batch_size() later folds those deltas into walk->lruvec under
>> the lruvec lock.
>>
>> The page table walker can run concurrently with the memcg reparenting path
>> as follows:
>>
>> CPU0                           CPU1
>> ====                           ====
>>
>> walk_mm
>> --> walk_page_range
>>      --> update_batch_size
>>          --> walk->nr_pages += delta
>>
>>                                mem_cgroup_css_offline
>>                                --> memcg_reparent_objcgs
>>                                    --> lock lruvec
>>                                        lru_gen_reparent_memcg
>>                                        --> reparent child folios to parent
>>                                        unlock lruvec
>>
>>      lock lruvec
>>      reset_batch_size
>>      --> child lrugen->nr_pages += delta
>>
>> This will trigger the following warning in lru_gen_exit_memcg():
>>
>> 	VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
>> 				   sizeof(lruvec->lrugen.nr_pages)));
>>
>> And the user-visible impact of underestimated nr_pages in MGLRU was
>> premature OOMs because MGLRU does not try to reclaim memory when nr_pages
>> reaches zero, but there are still more pages.
>>
>> To fix it, make reset_batch_size() check CSS_DYING under RCU before
>> flushing the pending batch. A non-dying memcg keeps the original lruvec
>> stable against RCU-delayed offlining; a dying memcg redirects the deltas
>> to the first non-dying ancestor.
>>
>> Reported-by: Peiyang He <peiyang_he@smail.nju.edu.cn>
>> Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@smail.nju.edu.cn
>> Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>> Changes in v3:
>>   - re-implement lock_batch_lruvec() by checking CSS_DYING under the RCU lock
>>     (suggested by Harry)
>>   - update the commit message (suggested by Harry)
>>   - temporarily drop the previous Reviewed-by tags
>>     (since the sync method has changed)
>>   - rebase onto the next-20260624
>>
>> Changes in v2:
>>   - update the commit message (pointed by Barry)
>>   - collect Reviewed-by
>>
>>   mm/vmscan.c | 45 ++++++++++++++++++++++++++++++++++++++-------
>>   1 file changed, 38 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 35c3bb15ae96..1ec8c23c72b9 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -3262,10 +3262,44 @@ static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio,
>>   	walk->nr_pages[new_gen][type][zone] += delta;
>>   }
>>   
>> +#ifdef CONFIG_MEMCG
>> +static struct lruvec *lock_batch_lruvec(struct lruvec *lruvec)
> 
> This is memcg specific function, move this function next to similar functions
> like lruvec_lock_irq. Also put irq in the name.

Currently, the lock_batch_lruvec() is only used by reset_batch_size().
Are you intend to make it a common function for use in other places?

Perhaps we could defer making it generic until we actually have a second
user. Since this is just a fix, keeping it self-contained within
vmscan.c might be more compact for now. ;)

> 
> BTW have you checked other places where lruvec_lock_irq is used and if similar
> kind of situation can happen?

I just checked and found no such callers.

Thanks,
Qi

>