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

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Message-ID: <CAJj2-QHV09Rh+s2AgDtC815YUJUnYgTs0EE9YbDvXdPM9DsKug@mail.gmail.com>
Date: Mon, 15 Dec 2025 13:42:27 -0600
From: Yuanchu Xie <yuanchu@...gle.com>
To: Deepanshu Kartikey <kartikey406@...il.com>
Cc: 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 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