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

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Date: Wed, 3 Jan 2024 18:17:44 -0700
From: Yu Zhao <yuzhao@...gle.com>
To: Dan Schatzberg <schatzberg.dan@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	cgroups@...r.kernel.org, linux-mm@...ck.org, 
	Yosry Ahmed <yosryahmed@...gle.com>, Michal Hocko <mhocko@...e.com>, 
	David Rientjes <rientjes@...gle.com>, Chris Li <chrisl@...nel.org>, Tejun Heo <tj@...nel.org>, 
	Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>, 
	Jonathan Corbet <corbet@....net>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Shakeel Butt <shakeelb@...gle.com>, 
	Muchun Song <muchun.song@...ux.dev>, David Hildenbrand <david@...hat.com>, 
	Matthew Wilcox <willy@...radead.org>, Kefeng Wang <wangkefeng.wang@...wei.com>, 
	Yue Zhao <findns94@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v6 2/2] mm: add swapiness= arg to memory.reclaim

On Wed, Jan 3, 2024 at 11:20 AM Dan Schatzberg <schatzberg.dan@...il.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."