Re: [PATCH v4] mm: mglru: fix stale batch updates after memcg reparenting

[Harry Yoo]

On 7/1/26 11:57 PM, Usama Arif wrote:
> On Wed,  1 Jul 2026 15:52:51 +0800 Qi Zheng <qi.zheng@xxxxxxxxx> wrote:
> 
>> From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxxxxx>
>> Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@xxxxxxxxxxxxxxxx
>> Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios")
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
>> Reviewed-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
>> ---
>> Changes in v4:
>>  - re-implement lock_batch_lruvec() in a simpler way
>>    (suggested by Johannes and Harry)
>>  - collect Reviewed-by
>>  - rebase onto the next-20260630
>>
>> 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 | 41 ++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 34 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 35c3bb15ae96..ca1e2a870d51 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -3262,10 +3262,40 @@ 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)
>> +{
>> +	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>> +
>> +	rcu_read_lock();
>> +
>> +	/*
>> +	 * The memcg can be NULL when the memory controller is disabled.
>> +	 * Otherwise, the caller keeps the memcg owning @lruvec alive.
>> +	 */
>> +	while (unlikely(memcg && css_is_dying(&memcg->css))) {
>> +		memcg = parent_mem_cgroup(memcg);
>> +		lruvec = mem_cgroup_lruvec(memcg, pgdat);
>> +	}
>> +
>> +	spin_lock_irq(&lruvec->lru_lock);
> 
> Do we need an rcu_read_unlock() here?

lruvec_unlock_irq() does that.

-- 
Cheers,
Harry / Hyeonggon