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Received: from 30.74.144.127(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XAbBF0o_1788860384 cluster:ay36) by smtp.aliyun-inc.com; Tue, 08 Sep 2026 17:39:46 +0800 Message-ID: <7d2ee486-0f59-43b7-8449-fed82b796a73@linux.alibaba.com> Date: Tue, 8 Sep 2026 17:39:44 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: mglru: clear the reference counter for rejected folios To: Baoquan He Cc: Barry Song , akpm@linux-foundation.org, kasong@tencent.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, hannes@cmpxchg.org, david@kernel.org, mhocko@kernel.org, ljs@kernel.org, ridong.chen@linux.dev, hebaoquan@kylinos.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <8e4db9a298c5ea6ccb192e274caed5b96f0cf022.1788751143.git.baolin.wang@linux.alibaba.com> <67d9bbcf-2991-4867-8afb-14bad1cebf8d@linux.alibaba.com> <31494361-406e-4874-8a2c-2c6f5cee5321@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: E061C14000D X-Stat-Signature: 7a1ri96igews9fgfojkytrs8gywcmwq9 X-HE-Tag: 1788860392-144141 X-HE-Meta: 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 PKvVU8fw 7Ef5zdtizd067MDHzrTPrBLldINUF7TQhCyyGzOUoF5/uLMeIitESmNM4jyvHNClobV+Q+Ni+1MluwNCr9HnwzTChEZCWrtCURm4vC6Y4WDZ+WhbRLo3/pVsf3wfaXC7UIrVsI2McdkF/XWRCvzv423ovmaq+EuxMYqi3KIX2wH6/KlpL5EO6GTXCAZqeHojsQuS9DxjsQXpz79iL1N4ee2qAy5zjGqBWiXwvTMA2MiOacI5MgmFMFEaTh7/jkIYEmyApYjWceXh5zaXcw5g2keNoL6i/W+MsAt5ZJ0HsDaqnnneatwF12L0FD46a/OCpReSgfX64l+gVLcHJofWhBuEERMwHrv2sgU+P Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/8/26 4:23 PM, Baoquan He wrote: > On 09/08/26 at 03:53pm, Baolin Wang wrote: >> >> >> On 9/8/26 2:59 PM, Baoquan He wrote: >>> On 09/08/26 at 12:01pm, Baolin Wang wrote: >>>> >>>> >>>> On 9/8/26 11:03 AM, Baolin Wang wrote: >>>>> >>>>> >>>>> On 9/8/26 10:34 AM, Barry Song wrote: >>>>>> On Tue, Sep 8, 2026 at 10:30 AM Baoquan He wrote: >>>>>>> >>>>>>> Hi Baolin, >>>>>>> >>>>>>> On 09/07/26 at 11:25am, Baolin Wang wrote: >>>>>>> ......snip... >>>>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c >>>>>>>> index 40d3f1b48a74..42c0a09938ab 100644 >>>>>>>> --- a/mm/vmscan.c >>>>>>>> +++ b/mm/vmscan.c >>>>>>> >>>>>>> Well, this seems to be based on Andrew's mm-new branch. I usually track >>>>>>> mm-unstable branch. Maybe the subject should be marked as below? >>>>>>> [PATCH mm-new] mm: mglru: clear the reference counter for rejected >>>>> >>>>> ACK. >>>>> >>>>>>> >>>>>>>> @@ -5021,10 +5021,11 @@ static int evict_folios(unsigned >>>>>>>> long nr_to_scan, struct lruvec *lruvec, >>>>>>>>                } >>>>>>>> >>>>>>>>                /* don't add rejected folios to the oldest generation */ >>>>>>>> -             if (lru_gen_folio_seq(lruvec, folio, false) == >>>>>>>> min_seq[type]) { >>>>>>>> -                     folio_set_lru_refs(folio, 0); >>>>>>>> +             if (lru_gen_folio_seq(lruvec, folio, false) == >>>>>>>> min_seq[type]) >>>>>>>>                        folio_set_active(folio); >>>>>>>> -             } >>>>>>>> + >>>>>>>> +             /* See the comments on LRU_REFS_FLAGS */ >>>>>>>> +             folio_set_lru_refs(folio, 0); >>>>>>> >>>>>>> This looks like a great catch, while the code change could bring issue. >>>>>>> >>>>>>> Because move_folios_to_lru() relies on folios' flags to decide their new >>>>>>> generation. You just cleared it before move_folios_to_lru(). This is no >>>>>>> problem for rejected folios that are determined to be put into the >>>>>>> oldest generation. But for those rejected folios that are determined to >>>>>>> be promoted, this could be wrong. E.g currently gen window is 4, and a >>>>>>> folio is referenced, lru_gen_folio_seq() decides its new gen as 1, which >>>>>>> is the 2nd oldest generation. While folio_set_lru_refs(folio, 0) clear >>>>>>> referenced bit, this causes it being put into the oldest generation in >>>>>>> move_folios_to_lru(), this is not expected. >>>>> >>>>> Yes. As I discussed with Barry earlier, lru_gen_folio_seq() also needs >>>>> to be reconsidered regarding whether it should rely on PG_referenced >>>>> [1]. >>>>> >>>>> For commit 6cbdd9726fb5, we didn't discuss the impact on rejected folios >>>>> either. Before commit 6cbdd9726fb5, if rejected folios did not have >>>>> PG_active set by shrink_folio_list(), evict_folios() would set PG_active >>>>> on these rejected folios. >>>>> >>>>> [1] https://lore.kernel.org/linux-mm/20260901220430.79810-1- >>>>> baohua@kernel.org/ >>>>> >>>>>> The original code looks quite weird. It even prioritizes folios >>>>>> that won't be promoted by `PG_active`. Do we need to change all >>>>>> the cases just to call `PG_active`? >>> >>> I would agree if we can. While I have one concern. A rejected folio that >>> should have gone into the oldest generation is now being promoted to the >>> 2nd newest generation. In the original code, referenced folio is only >>> being promoted to the next gen. I even think this is not a bug, but Yu >> >> That's not quite true. Before commit 6cbdd9726fb5, a rejected referenced >> folio was also put back to the 2nd youngest gen. > > OK, I didn't follow your earlier discussion, I need take some time to > fully understand that commit and the patch thread from Ehab. > >> >>> Zhao intentionally did it: the coldest folio is moved to 2nd newest gen, >>> referenced folio (hot folio) is moved to new gen but carries the referenced >>> bit. Both of them seems to be treated somewhat equally. >>> >>> To me, I would rather move both of them to the next gen, while keep their >>> refs untouched. >> >> IMHO, I strongly recommend not doing this, and that's exactly the motivation >> behind my patch. Because this is also being treated as a promotion, it >> should behave like folio_inc_gen() or folio_update_gen() and clear the refs >> after promotion. I think this is a fundamental principle of promotion. >> Otherwise, ref-based promotion is already completely broken. > > OK, it makes sense to me to make principle of promotion strictly applied > no efficiency degradation involved. > >> >> Next, I also plan to clean up refs in lru_gen_set_refs() as discussed with >> Barry. > > Looks forward to seeing that. By the way, your discussion with Baryr is > private or in public list, do you have pointer if public? Thanks. I raised this issue before[1], and recently there has been another discussion[2] about it (we had some private discussions, but I'll post a new patch for discussion). [1] https://lore.kernel.org/all/eb395442-0aad-428a-a5ac-9072d2d89060@linux.alibaba.com/ [2] https://lore.kernel.org/linux-mm/20260901220430.79810-1-baohua@kernel.org/ >>>>> Yes. Regarding this concern, I plan to change back to the original >>>>> behavior: >>>>> >>>>>     /* See the comments on LRU_REFS_FLAGS */ >>>>>     folio_set_lru_refs(folio, 0); >>>>> >>>>>     /* don't add rejected folios to the oldest generation */ >>>>>     if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type]) >>>>>         folio_set_active(folio); >>>>> >>>>> What do you think? >>>> >>>> Just FYI, after above changes, the performance improvement on zram is no >>>> longer obvious either. I think this also answers Kairui's earlier question >>>> about why I saw a performance improvement (which seems related to commit >>>> 6cbdd9726fb5). Also, there is no obvious performance regression either. >>> >>> Hi Baolin, >>> >>> Not sure if it's convenient to do a little more testing in your side. >>> E.g rejected folios are moved to next gen, but not clearing their flags. >> >> I'm not sure what you mean by "next gen" here. If you mean the 2nd oldest >> gen, I actually tested that too, that is, clearing refs after >> move_folios_to_lru(), and there wasn't any noticeable performance impact >> either (but the code was a bit hacky, so I didn't go with this approach). > > Yeah, I meant the 2nd oldest gen, while what I am curious about is moving > them into 2nd oldest gen but not clearing refs. Clearly it's conflicting > with your plan. Anyway, it's just a brain store idea, please forget it. > Thanks for the sharing and detailed explanation. Thanks for reviewing.