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

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Message-Id: <243A0156-D26A-47C9-982A-C8B0CDD69DA2@linux.dev>
Date:   Fri, 1 Apr 2022 14:21:52 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     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@...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>,
        Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

> 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.
> 
> 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.