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

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

Hi Andrew and David,

Gentle ping.

Is this patch good to be applied?

Thanks,
Qi

On 7/1/26 3:52 PM, 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>
> Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
> ---
> 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
>