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

Yuanchu Xie
2025-12-15T19:42:45Z

On Sun, Dec 14, 2025 at 10:52 PM Deepanshu Kartikey
<kartikey406@gmail.com> wrote:
>
> On Mon, Dec 15, 2025 at 9:42 AM Johannes Weiner <hannes@cmpxchg.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