linux-kernel - Re: [PATCH cgroup/for-4.8] cgroup: allow NULL return from ss->css

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Message-ID: <20160621185740.GR3262@mtj.duckdns.org>
Date:	Tue, 21 Jun 2016 14:57:40 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Li Zefan <lizefan@...wei.com>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH cgroup/for-4.8] cgroup: allow NULL return from
 ss->css_alloc()

Hello,

On Tue, Jun 21, 2016 at 02:14:04PM -0400, Johannes Weiner wrote:
> Would it be better to remove the error code instead and have everybody
> return NULL? AFAICS, everybody is returning either the object or the
> -ENOMEM error code right now.
> 
> What error condition is there for an allocation function?

I already changed cgroup core to interpret NULL return as -ENOMEM.
Maybe we can drop ERR_PTR handling altogether in the future but I'm
not sure.  e.g. memcg's id allocation and -ENOSPC check could be in
css_alloc.  The thing is css_online is only necessary if there are
requirements around css id or iteration, so css_alloc can easily be
the only alloc / init point and thus should be able to indicate
different failure conditions.

Thanks.

-- 
tejun