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

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Date:   Mon, 4 Apr 2022 19:30:21 -0700
From:   Wei Xu <weixugc@...gle.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Cgroups <cgroups@...r.kernel.org>,
        "open list:DOCUMENTATION" <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 Mon, Apr 4, 2022 at 10:08 AM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Fri, Apr 1, 2022 at 1:14 PM Wei Xu <weixugc@...gle.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.