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

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 13 Dec 2025 14:06:39 +0530 Deepanshu Kartikey <kartikey406@gmail.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.