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From: Qi Zheng In-Reply-To: <5a0c6597-6b96-4781-a71b-fd1298b2b7bb@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 0113E1C0007 X-Rspam-User: X-Stat-Signature: atgbk7szueumhj93i4tatu8aczrepigb X-HE-Tag: 1782457508-491271 X-HE-Meta: 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 LALLlSNq ru1KSL1MV7BJnRbw3QhGSLOxzJL+wr+LuHyndEl++cmZ+JmVPY95Bg3rGhZyuXXONL3cH8+BknblHe0AWJ4GeCt3Xtwwkn/PTY15IqmVZ0+iW+Z2MXXBwR8NBCcIWt3bSbS8pyDL8mQxH2YnbB/m9R0mK0sBTtrP08654V4lFWIosUl4rvjFUdsO2XZVOKZ8zQLWVkDvp7ekbgEH1NZ0MQQwFYPoWPBTrcvC3Q17q1y8I8PljDCcpuqJ4uYY1CeuKVmGQFCtQYTAmwiMPpF1avqKJynweTgR/YMPX Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/26/26 2:48 PM, Harry Yoo wrote: > > > On 6/26/26 3:24 PM, Qi Zheng wrote: >> >> >> 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? > > I don't think there's a coding style guide that specifically > mentions this. Just thought it's cleaner because it merges if (...) goto > lock; and do-while into a single while loop. > >>> 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? > > But is worth saving a few instruction in a basic block that is > unlikely() to be executed? I don't have a strong opinion here. Hi Johannes, I'll leave the decision up to you. If necessary, I can send out the v4. > > I'm not a memcg maintainer myself, though. just my 2 cents. I'd like to express my gratitude for your reviews, and especially for your invaluable help with the earlier dying memcg work! Thanks, Qi >