Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

[Akinobu Mita]

2026年1月15日(木) 2:49 Gregory Price <gourry@xxxxxxxxxx>:
>
> On Wed, Jan 14, 2026 at 09:51:28PM +0900, Akinobu Mita wrote:
> > can_demote() is called from four places.
> > I tried modifying the patch to change the behavior only when can_demote()
> > is called from shrink_folio_list(), but the problem was not fixed
> > (oom did not occur).
> >
> > Similarly, changing the behavior of can_demote() when called from
> > can_reclaim_anon_pages(), shrink_folio_list(), and can_age_anon_pages(),
> > but not when called from get_swappiness(), did not fix the problem either
> > (oom did not occur).
> >
> > Conversely, changing the behavior only when called from get_swappiness(),
> > but not changing the behavior of can_reclaim_anon_pages(),
> > shrink_folio_list(), and can_age_anon_pages(), fixed the problem
> > (oom did occur).
> >
> > Therefore, it appears that the behavior of get_swappiness() is important
> > in this issue.
>
> "It appears that..." and the process of twiddling bits and observing
> behavior does not strike confidence in this solution.
>
> Can you take another go at trying to define the bad interaction more
> explicitly? I worry that we're modifying vmscan.c behavior to induce an
> OOM for a corner case - but it will also cause another regression.

I agree.
It surprised me that the behavior of get_swappiness() had an impact on the
issue, so I'll clarify its relationship to this issue.