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Thu, 09 Jul 2026 18:51:03 -0700 (PDT) Date: Thu, 9 Jul 2026 21:51:01 -0400 From: Johannes Weiner To: Qi Zheng Cc: Harry Yoo , Usama Arif , akpm@linux-foundation.org, david@kernel.org, kasong@tencent.com, shakeel.butt@linux.dev, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, muchun.song@linux.dev, peiyang_he@smail.nju.edu.cn, mhocko@kernel.org, roman.gushchin@linux.dev, ljs@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Qi Zheng , stable@vger.kernel.org Subject: Re: [PATCH v4] mm: mglru: fix stale batch updates after memcg reparenting Message-ID: References: <20260701145736.3785016-1-usama.arif@linux.dev> <2fb5ce53-666b-4b0a-a4ad-2b3a28c54768@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Jul 02, 2026 at 09:38:41AM +0800, Qi Zheng wrote: > > > On 7/1/26 11:59 PM, Johannes Weiner wrote: > > On Thu, Jul 02, 2026 at 12:36:42AM +0900, Harry Yoo wrote: > >> > >> > >> On 7/1/26 11:57 PM, Usama Arif wrote: > >>> On Wed, 1 Jul 2026 15:52:51 +0800 Qi Zheng wrote: > >>> > >>>> From: Qi Zheng > >>>> > >>>> 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 > >>>> 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: > >>>> Signed-off-by: Qi Zheng > >>>> Reviewed-by: Harry Yoo (Oracle) > >>>> --- > >>>> 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. > > > > Yeah, that tripped me up too. And it makes me think Shakeel was right > > after all: this should live next to the other lruvec_lock() primitives. > > > > Sure, MGLRU is the only user, but it's still much easier to understand > > this if the code sits next to the rest of the API (and the unlock!). > > > > lruvec_live_lock_irq()? > > But lruvec_lock_irq() grabs the rcu lock too. :( Yes, but it's self-explanatory if you put it with those definitions: static inline void lruvec_lock_irq(struct lruvec *lruvec) { rcu_read_lock(); spin_lock_irq(&lruvec->lru_lock); } static struct lruvec *lruvec_live_lock_irq(struct lruvec *lruvec) { struct mem_cgroup *memcg = lruvec_memcg(lruvec); rcu_read_lock(); while (unlikely(memcg && css_is_dying(&memcg->css))) { memcg = parent_mem_cgroup(memcg); lruvec = mem_cgroup_lruvec(memcg, lruvec_pgdat(lruvec)); } spin_lock_irq(&lruvec->lru_lock); } static inline void lruvec_unlock_irq(struct lruvec *lruvec) { spin_unlock_irq(&lruvec->lru_lock); rcu_read_unlock(); } etc.