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From:   Wei Xu <weixugc@google.com>
Date:   Fri, 1 Apr 2022 14:38:53 -0700
Message-ID: <CAAPL-u_UiJQetHJbMXb6CbgzjLOUUFGcLnLn0MDGTN=LWpcgqQ@mail.gmail.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Yosry Ahmed <yosryahmed@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, Jonathan Corbet <corbet@lwn.net>,
        Yu Zhao <yuzhao@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Greg Thelen <gthelen@google.com>
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On Fri, Apr 1, 2022 at 2:21 PM Roman Gushchin <roman.gushchin@linux.dev> wr=
ote:
>
> > On Apr 1, 2022, at 2:13 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > =EF=BB=BFOn Fri, Apr 01, 2022 at 11:39:30AM -0700, Roman Gushchin wrote=
:
> >> The interface you're proposing is not really extensible, so we'll like=
ly need to
> >> introduce a new interface like memory.reclaim_ext very soon. Why not c=
reate
> >> an extensible API from scratch?
> >>
> >> I'm looking at cgroup v2 documentation which describes various interfa=
ce files
> >> formats and it seems like given the number of potential optional argum=
ents
> >> the best option is nested keyed (please, refer to the Interface Files =
section).
> >>
> >> E.g. the format can be:
> >> echo "1G type=3Dfile nodemask=3D1-2 timeout=3D30s" > memory.reclaim
> >
> > Yeah, that syntax looks perfect.
> >

I agree this is a better syntax than positional arguments. The latter
would require a default value be specified for each earlier argument
if we just want to provide a custom value for a later argument.

> > But why do you think it's not extensible from the current patch? We
> > can add those arguments one by one as we agree on them, and return
> > -EINVAL if somebody passes an unknown parameter.
> >
> > It seems to me the current proposal is forward-compatible that way
> > (with the current set of keyword pararms being the empty set :-))
>
> It wasn=E2=80=99t obvious to me. We spoke about positional arguments and =
then it wasn=E2=80=99t clear how to add them in a backward-compatible way. =
The last thing we want is a bunch of memory.reclaim* interfaces :)
> So yeah, let=E2=80=99s just describe it properly in the documentation, no=
 code changes are needed.