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

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Message-Id: <20251214154944.6a6e91ffb00677a5164074e9@linux-foundation.org>
Date: Sun, 14 Dec 2025 15:49:44 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Deepanshu Kartikey <kartikey406@...il.com>
Cc: axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
 hannes@...xchg.org, 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 Sat, 13 Dec 2025 14:06:39 +0530 Deepanshu Kartikey <kartikey406@...il.com> wrote:

> When laptop_mode is enabled, may_writepage is set to 0 in
> try_to_free_mem_cgroup_pages(). This triggers a warning in MGLRU's
> lru_gen_shrink_lruvec():
> 
>     VM_WARN_ON_ONCE(!sc->may_writepage || !sc->may_unmap);
> 
> The warning occurs because MGLRU expects full reclaim capabilities to
> function correctly. The call path is:
> 
>     mem_cgroup_resize_max()
>       try_to_free_mem_cgroup_pages()
>         do_try_to_free_pages()
>           shrink_node()
>             shrink_lruvec()
>               lru_gen_shrink_lruvec()  <-- WARNING
> 
> Unlike kswapd or direct reclaim where laptop_mode's disk-saving behavior
> is a reasonable optimization, memcg limit enforcement is a hard
> requirement - memory MUST be freed when a cgroup exceeds its limit.
> The may_unmap field is already set unconditionally to 1 in this path,
> acknowledging that memcg reclaim needs full capabilities.
> 
> Set may_writepage unconditionally to 1 for memcg reclaim to ensure
> MGLRU works correctly and memory limits are properly enforced.
> 

Thanks, I'll add this to mm.git's mm-new branch for testing.  I expect
a few days after that I'll quietly move it into the mm-unstable branch
where it will receive linux-next exposure.  Further progress into
mm.git's non-rebasing for-next-merge-window mm-stable branch will
depend upon review outcomes.