Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.35 as permitted sender) client-ip=23.128.96.35;
MIME-Version: 1.0
References: <20231211140419.1298178-1-schatzberg.dan@gmail.com>
 <20231211140419.1298178-2-schatzberg.dan@gmail.com> <CAJD7tkZQ2aakT8M2bTg0bp4sDtrGYv_4i4Z4z3KBerfxZ9qFWA@mail.gmail.com>
 <ZXjQLXJViHX7kMnV@dschatzberg-fedora-PF3DHTBV>
In-Reply-To: <ZXjQLXJViHX7kMnV@dschatzberg-fedora-PF3DHTBV>
From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Tue, 12 Dec 2023 13:31:46 -0800
Message-ID: <CAJD7tkY6iTxKe4Nj00sN3vh26Cw2Sw+kTkY40r000+ttN-dFpg@mail.gmail.com>
Subject: Re: [PATCH V3 1/1] mm: add swapiness= arg to memory.reclaim
To:     Dan Schatzberg <schatzberg.dan@gmail.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        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>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Chris Li <chrisl@kernel.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        "Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
        Yue Zhao <findns94@gmail.com>, Hugh Dickins <hughd@google.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, Dec 12, 2023 at 1:27=E2=80=AFPM Dan Schatzberg <schatzberg.dan@gmai=
l.com> wrote:
>
> On Mon, Dec 11, 2023 at 11:41:24AM -0800, Yosry Ahmed wrote:
> > On Mon, Dec 11, 2023 at 6:04=E2=80=AFAM Dan Schatzberg <schatzberg.dan@=
gmail.com> wrote:
> >
> > contains* the*
> >
> > I think this statement was only important because no keys were
> > supported, so I think we can remove it completely and rely on
> > documenting the supported keys below like other interfaces, see my
> > next comment.
> >
> > > +       to reclaim.
> > >
> > >         Example::
> > >
> > > @@ -1304,6 +1304,17 @@ PAGE_SIZE multiple when read back.
> > >         This means that the networking layer will not adapt based on
> > >         reclaim induced by memory.reclaim.
> > >
> > > +       This file also allows the user to specify the swappiness valu=
e
> > > +       to be used for the reclaim. For example:
> > > +
> > > +         echo "1G swappiness=3D60" > memory.reclaim
> > > +
> > > +       The above instructs the kernel to perform the reclaim with
> > > +       a swappiness value of 60. Note that this has the same semanti=
cs
> > > +       as the vm.swappiness sysctl - it sets the relative IO cost of
> > > +       reclaiming anon vs file memory but does not allow for reclaim=
ing
> > > +       specific amounts of anon or file memory.
> > > +
> >
> > Can we instead follow the same format used by other nested-keyed files
> > (e.g. io.max)? This usually involves a table of supported keys and
> > such.
>
> Thanks, both are good suggestions. Will address these.
>
> > > +       while ((start =3D strsep(&buf, " ")) !=3D NULL) {
> > > +               if (!strlen(start))
> > > +                       continue;
> > > +               switch (match_token(start, if_tokens, args)) {
> > > +               case MEMORY_RECLAIM_SWAPPINESS:
> > > +                       if (match_int(&args[0], &swappiness))
> > > +                               return -EINVAL;
> > > +                       if (swappiness < 0 || swappiness > 200)
> >
> > I am not a fan of extending the hardcoded 0 and 200 values, and now
> > the new -1 value. Maybe it's time to create constants for the min and
> > max swappiness values instead of hardcoding them everywhere? This can
> > be a separate preparatory patch. Then, -1 (or any invalid value) can
> > also be added as a constant with a useful name, instead of passing -1
> > to all other callers.
> >
> > This should make the code a little bit more readable and easier to exte=
nd.
>
> I'm not sure I understand the concern. This check just validates that
> the swappiness value inputted is between 0 and 200 (inclusive)
> otherwise the interface returns -EINVAL. Are you just concerned that
> these constants are not named explicitly so they can be reused
> elsewhere in the code?

Yes. The 0 and 200 values are already hardcoded in multiple places,
and we are adding more places now and more hardcoded values (i.e. -1).