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From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: z9tmptbe97g3dqzwtiwupc9sjg7nuq9w X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D39B010000E X-HE-Tag: 1782455082-159134 X-HE-Meta: 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 +X/Ioduv Th3zB7Xx7km7E7CT9phOl1c82Jdgu21JvwR+ZmF066+KYD/JWXBfdK5Xh1Cpkus891tFD3NgpiSiBoeM599LOueQuKJjk8uURcEbI8y7IR2scYx6CbXQjGUuornHo8AxkX9MM4U1b1Olu2rSd5MeaWS5MMIxIeY3z67LRagzuOOx9NWcBJ6mgYXnna1O7mMvPzckm30uMJzHcpYEwPnNr2B/Kko13TTGp5ldPh3Om9ZO7t03Cu6761f9e6iZFczX2pxWw Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/26/26 12:59 PM, Harry Yoo wrote: > > > On 6/26/26 1:48 PM, Qi Zheng wrote: >> >> >> On 6/26/26 12:43 PM, Harry Yoo wrote: >>> >>> >>> On 6/26/26 11:27 AM, Qi Zheng wrote: >>>> Hi Johannes, >>>> >>>> On 6/26/26 2:41 AM, Johannes Weiner wrote: >>>>> On Thu, Jun 25, 2026 at 11:15:54PM +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 >>>>>> --- >>>>>> 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(); >>>>> >>>>> Where is this unlocked? >>>> >>>> The lruvec_unlock_irq() in reset_batch_size() will handle the unlocking. >>>> >>>>> >>>>>> + /* >>>>>> + * 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); >>>>> >>>>> while (unlikely(memcg && css_is_dying(&memcg->css))) { >>>>> memcg = parent_mem_cgroup(memcg); >>>>> lruvec = mem_cgroup_lruvec(memcg, pgdat); >>>> >>>> There is no need to acquire the lruvec before finding the first >>>> non-dying memcg. >>> >>> struct pglist_data *pgdat = lruvec_pgdat(lruvec); >>> struct mem_cgroup *memcg = lruvec_memcg(lruvec); >>> >>> rcu_read_lock() >>> >>> while (unlikely(memcg_is_dying(memcg))) >>> memcg = parent_mem_cgroup(memcg); >>> >>> lruvec = mem_cgroup_lruvec(memcg, pgdat); >> >> If the first memcg is already non-dying, there's no need to re-acquire >> the lruvec. ;) > > Oh, right :) > > Hmm but I still think Johannes' suggestion makes the code cleaner. I don't have a strong preference on which of the two coding styles is more readable. BTW, is there any kernel documentation I could refer to for this? > Observing a dying cgroup should be rare anyway, it's worth focusing > more on readability? While it's rare to encounter consecutive dying memcgs, it can still happen, right? Thanks, Qi >