Re: [PATCH v6 2/2] mm: add swapiness= arg to memory.reclaim

[Yu Zhao <yuzhao@google.com>]

On Wed, Jan 3, 2024 at 11:20 AM Dan Schatzberg <schatzberg.dan@gmail.com> wrote:
>
> On Wed, Jan 03, 2024 at 10:19:40AM -0700, Yu Zhao wrote:
> [...]
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index d91963e2d47f..394e0dd46b2e 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -92,6 +92,11 @@ struct scan_control {
> > >         unsigned long   anon_cost;
> > >         unsigned long   file_cost;
> > >
> > > +#ifdef CONFIG_MEMCG
> > > +       /* Swappiness value for proactive reclaim. Always use sc_swappiness()! */
> > > +       int *proactive_swappiness;
> > > +#endif
> >
> > Why is proactive_swappiness still a pointer? The whole point of the
> > previous conversation is that sc->proactive can tell whether
> > sc->swappiness is valid or not, and that's less awkward than using a
> > pointer.
>
> It's the same reason as before - zero initialization ensures that the
> pointer is NULL which tells us if it's valid or not. Proactive reclaim
> might not set swappiness and you need to distinguish swappiness of 0
> and not-set. See this discussion with Michal:
>
> https://lore.kernel.org/linux-mm/ZZUizpTWOt3gNeqR@tiehlicka/
>
> > Also why the #ifdef here? I don't see the point for a small stack
> > variable. Otherwise wouldn't we want to do this for sc->proactive as
> > well?
>
> This was Michal's request and it feels similar to your rationale for
> naming it proactive_swappiness - it's just restricting the interface
> down to the only use-cases. I'd be fine with doing the same in
> sc->proactive as a subsequent patch.
>
> See https://lore.kernel.org/linux-mm/ZZUhBoTNgL3AUK3f@tiehlicka/

Also regarding #ifdef, quoting Documentation/process/4.Coding.rst:
"As a general rule, #ifdef use should be confined to header files
whenever possible."