Re: [PATCH v2] mm: mglru: fix stale batch updates after memcg reparenting

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

On 6/23/26 10:42 AM, 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)));
> 
> To fix it, add lrugen->reparented to remember the new owner of a
> reparented lruvec, and make reset_batch_size() charge pending deltas to
> that owner.
> 
> 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: Barry Song <baohua@kernel.org>
> ---

Changes in v2:
  - update the commit message (pointed by Barry)
  - collect Reviewed-by

>   include/linux/mmzone.h |  4 ++++
>   mm/vmscan.c            | 43 +++++++++++++++++++++++++++++++++++-------
>   2 files changed, 40 insertions(+), 7 deletions(-)
>