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From: Qi Zheng In-Reply-To: <46ac28bf-5be1-4600-b522-0a1aa76c28e6@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 85AD61C0004 X-Stat-Signature: oiiixiwk68ax6iksxefgzzuz8fcroc8z X-HE-Tag: 1782449330-578143 X-HE-Meta: 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 6tALG11Y F5rXZvuTwEShrXVYRtu+KSP4W8GPXNjrBImZr/jHNxB3oL7NpSEtdPFxlA1onkOwsL7Xa9tJNyUWbfeX36lemuz2s0jvCUIGAbevTGezfYmkhK4X//xnXdEco+2L0xSUmqiYM8SqN0Nd8fvp6NPJS6JJZFsKT85ebQbu/TvLJmEynaZGkBiVnUcE1RmiHE4B3BGsCL5zlLpZoTTSBb+6/rOG8iBgBVW5sgZiyZL+7fDS/RLtJLdQr17lcEJyXVdE20tfi6H0eIOaXdAlfvj0QHs1KgwUCJcBj7PVd Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/26/26 12:43 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. ;) Thanks, Qi > spin_lock_irq(&lruvec->lru_lock); > > return lruvec; > > should work? > > if the memory controller is disabled, it's equivalent to: > > rcu_read_lock(); > spin_lock_irq(&lruvec->lru_lock); > return lruvec; >