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Date: Tue, 2 Jan 2024 10:21:44 -0500
From: Dan Schatzberg <schatzberg.dan@gmail.com>
To: Yu Zhao <yuzhao@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Yosry Ahmed <yosryahmed@google.com>, Huan Yang <link@vivo.com>,
	linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
	linux-mm@kvack.org, Tejun Heo <tj@kernel.org>,
	Zefan Li <lizefan.x@bytedance.com>,
	Jonathan Corbet <corbet@lwn.net>, Michal Hocko <mhocko@kernel.org>,
	Shakeel Butt <shakeelb@google.com>,
	Muchun Song <muchun.song@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kefeng Wang <wangkefeng.wang@huawei.com>,
	SeongJae Park <sj@kernel.org>,
	"Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
	Nhat Pham <nphamcs@gmail.com>, Yue Zhao <findns94@gmail.com>
Subject: Re: [PATCH v5 2/2] mm: add swapiness= arg to memory.reclaim
Message-ID: <ZZQqCHmocwUFvuTz@dschatzberg-fedora-PF3DHTBV>
References: <20231220152653.3273778-1-schatzberg.dan@gmail.com>
 <20231220152653.3273778-3-schatzberg.dan@gmail.com>
 <CAOUHufYwPzZ7k=ecFkxaw+26hUkiTODEnmKM8b3=Lk=n+bm29w@mail.gmail.com>
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In-Reply-To: <CAOUHufYwPzZ7k=ecFkxaw+26hUkiTODEnmKM8b3=Lk=n+bm29w@mail.gmail.com>

Hi Yu Zhao,

Thanks for the feedback, sorry for the delayed response.

On Thu, Dec 21, 2023 at 10:31:59PM -0700, Yu Zhao wrote:
> On Wed, Dec 20, 2023 at 8:27 AM Dan Schatzberg <schatzberg.dan@gmail.com> wrote:
> >
> > ...
> 
> The cover letter says:
> "Previously, this exact interface addition was proposed by Yosry[3]."
> 
> So I think it should be acknowledged with a Suggested-by, based on:
> "A Suggested-by: tag indicates that the patch idea is suggested by the
> person named and ensures credit to the person for the idea."
> from
> https://docs.kernel.org/process/submitting-patches.html#using-reported-by-tested-by-reviewed-by-suggested-by-and-fixes

Sure, will do.

> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index d91963e2d47f..aa5666842c49 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -92,6 +92,9 @@ struct scan_control {
> >         unsigned long   anon_cost;
> >         unsigned long   file_cost;
> >
> > +       /* Swappiness value for reclaim. NULL will fall back to per-memcg/global value */
> > +       int *swappiness;
> 
> Using a pointer to indicate whether the type it points to is
> overridden isn't really a good practice.
> 
> A better alternative was suggested during the v2:
> "Perhaps the negative to avoid unnecessary dereferences."
> https://lore.kernel.org/linux-mm/dhhjw4h22q4ngwtxmhuyifv32zjd6z2relrcjgnxsw6zys3mod@o6dh5dy53ae3/

I did have a couple versions with a negative but it creates
initialization issues where every instantiation of scan_control needs
to make sure to initialize swappiness or else it will behave as if
swappiness is 0. That's pretty error prone so using the pointer seemed
the better approach.

> Since only proactive reclaim can override swappiness, meaning it only
> happens if sc->proactive is true, I think the best way to make it work
> without spending much effort is create a helper as Michal suggest but
> it should look like:
> 
> sc_swappiness()
> {
>   return sc->proactive ? sc->swappiness : mem_cgroup_swappiness(memcg);
> }
> 
> In this patchset, sc->swappiness really means
> sc->proactive_swappiness. So it should be renamed accordingly.

Helper aside, I disagree with this point about coupling with the
proactive flag. The fact that the only user currently is proactive
reclaim doesn't imply to me that the interface (in scan_control)
should be coupled to the use-case. It's easier to reason about a
swappiness field that overrides swappiness for all scans that set it
regardless of the users.