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References: <20240103164841.2800183-1-schatzberg.dan@gmail.com>
 <20240103164841.2800183-3-schatzberg.dan@gmail.com> <ZZaDw1Fak_q9BnW-@tiehlicka>
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From: Yu Zhao <yuzhao@google.com>
Date: Tue, 9 Jan 2024 16:57:49 -0700
Message-ID: <CAOUHufZsf0WYQcUyMz7EcyvFYpaL4wLDZBW8oz9CgB5qZqSGAQ@mail.gmail.com>
Subject: Re: [PATCH v6 2/2] mm: add swapiness= arg to memory.reclaim
To: Michal Hocko <mhocko@suse.com>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, 
	linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, 
	Yosry Ahmed <yosryahmed@google.com>, David Rientjes <rientjes@google.com>, 
	Chris Li <chrisl@kernel.org>, Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>, 
	Johannes Weiner <hannes@cmpxchg.org>, Jonathan Corbet <corbet@lwn.net>, 
	Roman Gushchin <roman.gushchin@linux.dev>, Shakeel Butt <shakeelb@google.com>, 
	Muchun Song <muchun.song@linux.dev>, David Hildenbrand <david@redhat.com>, 
	Matthew Wilcox <willy@infradead.org>, Kefeng Wang <wangkefeng.wang@huawei.com>, 
	Yue Zhao <findns94@gmail.com>, Hugh Dickins <hughd@google.com>
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On Thu, Jan 4, 2024 at 3:09=E2=80=AFAM Michal Hocko <mhocko@suse.com> wrote=
:
>
> On Wed 03-01-24 08:48:37, Dan Schatzberg 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_swappine=
ss()! */
> > +     int *proactive_swappiness;
> > +#endif
> > +
> >       /* Can active folios be deactivated as part of reclaim? */
> >  #define DEACTIVATE_ANON 1
> >  #define DEACTIVATE_FILE 2
> > @@ -227,6 +232,13 @@ static bool writeback_throttling_sane(struct scan_=
control *sc)
> >  #endif
> >       return false;
> >  }
> > +
> > +static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *m=
emcg)
> > +{
> > +     if (sc->proactive && sc->proactive_swappiness)
> > +             return *sc->proactive_swappiness;
> > +     return mem_cgroup_swappiness(memcg);
> > +}
>
> If you really want to make this sc->proactive bound then do not use
> CONFIG_MEMCG as sc->proactive is not guarded either.
>
> I do not think that sc->proactive check is really necessary. A pure NULL
> check is sufficient to have a valid and self evident code that is future
> proof. But TBH this is not the most important aspect of the patch to
> spend much more time discussing. Either go with sc->proactive but make
> it config space consistent or simply rely on NULL check (with or without
> MEMCG guard as both are valid options).

Now you see why I replied. That "hybrid" if statement is just neither
of what was suggested.