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

[Harry Yoo]

On 6/23/26 11:42 AM, Qi Zheng 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.

Ouch.

IIRC 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.

Perhaps worth mentioning in the changelog?

> 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

The problem here is that, while grabbing a reference to memcg
(via mem_cgroup_iter(), for example) makes sure that the memcg is not
freed, it does not prevent offlining happening, and reset_batch_size()
doesn't check whether the lruvec has been reparented, or the lruvec
is going to be reparented.

> 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.

Could you please explain why it is unavoidable to introduce the new
field and why checking whether the cgroup is dying (and charging deltas
to non-dying parent) doesn't work?

> 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: Barry Song <baohua@xxxxxxxxxx>
> ---

-- 
Cheers,
Harry / Hyeonggon

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