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From:   Wei Xu <weixugc@google.com>
Date:   Mon, 4 Apr 2022 19:30:21 -0700
Message-ID: <CAAPL-u-7Vnq8tmmRNDOz7Jw1ei_Vw=idJaPzMPoq5=sBrpyrRA@mail.gmail.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Yosry Ahmed <yosryahmed@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Cgroups <cgroups@vger.kernel.org>,
        "open list:DOCUMENTATION" <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 Mon, Apr 4, 2022 at 10:08 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Fri, Apr 1, 2022 at 1:14 PM Wei Xu <weixugc@google.com> wrote:
> >
> [...]
> >
> > -EAGAIN sounds good, too.  Given that the userspace requests to
> > reclaim a specified number of bytes, I think it is generally better to
> > tell the userspace whether the request has been successfully
> > fulfilled. Ideally, it would be even better to return how many bytes
> > that have been reclaimed, though that is not easy to do through the
> > cgroup interface.
>
> What would be the challenge on returning the number of bytes reclaimed
> through cgroup interface?

write() syscall is used to write the command into memory.reclaim,
which should return either the number of command bytes written or -1
(errno is set to indicate the actual error).  I think we should not
return the number of bytes reclaimed through write().  A new
sys_reclaim() is better in this regard because we can define its
return value, though it would need a cgroup argument, which is not
commonly defined for syscalls.