linux-kernel - Re: [PATCH] mm: vmscan: always allow writeback during memcg reclaim

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Message-ID: <CAMgjq7BUJ3bS6bZ0__=RxMbFRfNk+1xpeZibGZy3j-gdWajO3g@mail.gmail.com>
Date: Fri, 19 Dec 2025 13:13:53 +0800
From: Kairui Song <ryncsn@...il.com>
To: Yuanchu Xie <yuanchu@...gle.com>
Cc: Deepanshu Kartikey <kartikey406@...il.com>, Johannes Weiner <hannes@...xchg.org>, akpm@...ux-foundation.org, 
	axelrasmussen@...gle.com, weixugc@...gle.com, david@...nel.org, 
	mhocko@...nel.org, zhengqi.arch@...edance.com, shakeel.butt@...ux.dev, 
	lorenzo.stoakes@...cle.com, yuzhao@...gle.com, heftig@...hlinux.org, 
	oleksandr@...alenko.name, bgeffon@...gle.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+90fcab4d88cffed6d0d8@...kaller.appspotmail.com
Subject: Re: [PATCH] mm: vmscan: always allow writeback during memcg reclaim

On Tue, Dec 16, 2025 at 3:52 AM Yuanchu Xie <yuanchu@...gle.com> wrote:
>
> On Sun, Dec 14, 2025 at 10:52 PM Deepanshu Kartikey
> <kartikey406@...il.com> wrote:
> >
> > On Mon, Dec 15, 2025 at 9:42 AM Johannes Weiner <hannes@...xchg.org> wrote:
> >
> > > > Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
> > >
> > > That seems unrelated?
> >
> > Sorry for the wrong fixes. Correct  Fixes: ee814fe23daf ("mm: vmscan:
> > clean up struct scan_control")
> >
> > I'll wait for input from someone with MGLRU context on the broader discussion.
> >
> This warning came from commit e9d4e1ee7880 ("mm: multi-gen LRU:
> clarify scan_control flags") [1].
>
> The original rationale:
> > 4. sc->may_writepage and sc->may_unmap, which indicates opportunistic
> >   reclaim, are rejected, since unmapped clean folios are already
> >   prioritized. Scanning for more of them is likely futile and can
> >   cause high reclaim latency when there is a large number of memcgs.
>
> As far as I can tell this was a sanity check to ensure
> `lru_gen_shrink_lruvec` avoids extra work for minimal gain. Perhaps
> this shouldn't be a warning? Always setting may_writepage in this case
> would free more folios. I'm not against removing the warning either.
>
> @Wei Xu @Axel Rasmussen  Any opinions?
>
> [1] https://lore.kernel.org/all/20221222041905.2431096-8-yuzhao@google.com/T/#u
>

Hi All,

We are also hitting this warning in our test environment. Simply
removing that WARN seems OK for us, shrink_folio_list will bounce
these folios back and everything behaves just fine.

Meanwhile, is it a good idea to add back the !sc->may_unmap check in
isolate_folio or improve isolate_folios accordingly? That might help
reduce the overhead in the worst scenarios.