From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-100.freemail.mail.aliyun.com (out30-100.freemail.mail.aliyun.com [115.124.30.100]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AD8B04D2EFE for ; Tue, 8 Sep 2026 09:39:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.100 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788860394; cv=none; b=XrysZCEnp+jMNd9U8K02BPDHZsTyBmK9F3zRkfCpKRNqX/lMwqsYAIaVFDSHDEBM6EzFAc6f6g3vdrV5D8uKeP12riSnERYTV0eqjKENAhY0tTs4SQ6nK/iYbeOF3f7Slvlpy3nNk7fcdH9oiW2wy0HaLdKZA5QLds2+U+dftXY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788860394; c=relaxed/simple; bh=yCew8vzP/YIvibWdDAJ7BqtfIX11EBzOyl+uk4tqf+A=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gCWK5KaW/RKjeL5oLhcTiARf5FHwtQEKTRVE2mq/JiDbADBUSvg6bK4FIjKPVjPlcBGSVzWeuJAqVqnIBzmM29skfjcbCGDWQGuFgye8ApQUH7G7p16ktInLh0jOZeeJARS88d+82xCGmTq6+zN9LAfz9OUA1NINNLBB5Kyc14w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=w9m8GmV+; arc=none smtp.client-ip=115.124.30.100 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="w9m8GmV+" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1788860387; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=PWYEPdRQ17N+5c66Weq133KixewGNO1R8mkZenc6WbY=; b=w9m8GmV+E+jZUftAfdD81heihklgeZWk7WOxtiq8HmbNLIkgrigOqkC493QgPuJBpiqQTnN7VpbaqokVn9cIjL6nB5j80pwHxz7qsgr/uRvq5IiXxj4OyZTc7RRkr6ILoiwG7Aarl2uOh7NDEtXiFcsM75kpNM1f9GhjwjodXTc= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R211e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033032089153;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=17;SR=0;TI=SMTPD_---0XAbBF0o_1788860384; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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.