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

[Peiyang He]

Hi Qi,

I have applied this patch and tested it against the PoC, the warning is not observed anymore in the kernel log.
Thanks for your nice work!

Best,
Peiyang

On 2026/6/25 23:15, 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.
> 
> 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>
> ---
> 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)
> +{
> +	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.
> +	 */
> +	if (!memcg || !css_is_dying(&memcg->css))
> +		goto lock;
> +
> +	do {
> +		memcg = parent_mem_cgroup(memcg);
> +	} while (memcg && css_is_dying(&memcg->css));
> +	lruvec = mem_cgroup_lruvec(memcg, pgdat);
> +
> +lock:
> +	spin_lock_irq(&lruvec->lru_lock);
> +
> +	return lruvec;
> +}
> +#else
> +static struct lruvec *lock_batch_lruvec(struct lruvec *lruvec)
> +{
> +	lruvec_lock_irq(lruvec);
> +
> +	return lruvec;
> +}
> +#endif
> +
>  static void reset_batch_size(struct lru_gen_mm_walk *walk)
>  {
>  	int gen, type, zone;
> -	struct lruvec *lruvec = walk->lruvec;
> +	struct lruvec *lruvec = lock_batch_lruvec(walk->lruvec);
>  	struct lru_gen_folio *lrugen = &lruvec->lrugen;
>  
>  	walk->batched = 0;
> @@ -3285,6 +3319,8 @@ static void reset_batch_size(struct lru_gen_mm_walk *walk)
>  			lru += LRU_ACTIVE;
>  		__update_lru_size(lruvec, lru, zone, delta);
>  	}
> +
> +	lruvec_unlock_irq(lruvec);
>  }
>  
>  static int should_skip_vma(unsigned long start, unsigned long end, struct mm_walk *args)
> @@ -3779,11 +3815,8 @@ static void walk_mm(struct mm_struct *mm, struct lru_gen_mm_walk *walk)
>  			mmap_read_unlock(mm);
>  		}
>  
> -		if (walk->batched) {
> -			lruvec_lock_irq(lruvec);
> +		if (walk->batched)
>  			reset_batch_size(walk);
> -			lruvec_unlock_irq(lruvec);
> -		}
>  
>  		cond_resched();
>  	} while (err == -EAGAIN);
> @@ -4867,9 +4900,7 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	walk = current->reclaim_state->mm_walk;
>  	if (walk && walk->batched) {
>  		walk->lruvec = lruvec;
> -		lruvec_lock_irq(lruvec);
>  		reset_batch_size(walk);
> -		lruvec_unlock_irq(lruvec);
>  	}
>  
>  	mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(sc),