Re: [PATCH -v4 1/4] mm/vmscan: fix anon-only reclaim evicting file pages when swappiness=max

[Ridong Chen]

On 7/27/2026 6:56 PM, Barry Song wrote:
> On Mon, Jul 27, 2026 at 4:19 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > On Fri 24-07-26 11:34:32, Ridong wrote:
> > > From: Ridong Chen <chenridong@xxxxxxxxxx>
> > >
> > > As Qi mentioned [1], when swappiness=max (SWAPPINESS_ANON_ONLY) is set,
> > > the reclaim logic is expected to reclaim anonymous pages exclusively.
> > > However, due to the current ordering of checks in get_scan_count(),
> > > file pages may still be evicted if can_reclaim_anon_pages() returns
> > > false, which contradicts the semantics of SWAPPINESS_ANON_ONLY.
> >
> > But if can_reclaim_anon_pages tells us that anonymous pages are not
> > reclaimable then it makes no sense to try that, no? What do you expect
> > to see? An OOM killer?
>
> This is proactive reclaim rather than direct reclaim, and
> SWAPPINESS_ANON_ONLY is only supported for proactive reclaim. It
> therefore cannot invoke the OOM killer.
>
> If I understand correctly, this is exactly the issue this patch is
> trying to address. Instead of falling back to reclaiming file folios,
> it returns "bash: echo: write error: Resource temporarily unavailable".
>
>   After (file cache is left intact):
>      # cat memory.stat
>      anon 200704
>      file 67178496
>      pgscan_proactive 0
>      # echo "64M swappiness=max" > memory.reclaim
>      -bash: echo: write error: Resource temporarily unavailable
>      # cat memory.stat
>      anon 208896
>      file 67178496             <- page cache untouched
>      pgsteal_proactive 0
>      pgscan_proactive 0
>
> Ridong, please correct me if I am wrong.

Thank you, Barry, it's correct.

--
Best regards
Ridong