linux-kernel - Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

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Date:   Fri, 1 Apr 2022 14:38:53 -0700
From:   Wei Xu <weixugc@...gle.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Jonathan Corbet <corbet@....net>,
        Yu Zhao <yuzhao@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

On Fri, Apr 1, 2022 at 2:21 PM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>
> > On Apr 1, 2022, at 2:13 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> >
> > On Fri, Apr 01, 2022 at 11:39:30AM -0700, Roman Gushchin wrote:
> >> The interface you're proposing is not really extensible, so we'll likely need to
> >> introduce a new interface like memory.reclaim_ext very soon. Why not create
> >> an extensible API from scratch?
> >>
> >> I'm looking at cgroup v2 documentation which describes various interface files
> >> formats and it seems like given the number of potential optional arguments
> >> the best option is nested keyed (please, refer to the Interface Files section).
> >>
> >> E.g. the format can be:
> >> echo "1G type=file nodemask=1-2 timeout=30s" > 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’t obvious to me. We spoke about positional arguments and then it wasn’t 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’s just describe it properly in the documentation, no code changes are needed.