linux-kernel - Re: [PATCH v2 0/3] cgroup: add xattr support

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Date:	Tue, 6 Mar 2012 13:37:36 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Li Zefan <lizf@...fujitsu.com>
cc:	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lennart Poettering <mzxreary@...inter.de>,
	Kay Sievers <kay.sievers@...y.org>,
	Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH v2 0/3] cgroup: add xattr support

On Sun, 4 Mar 2012, Tejun Heo wrote:
> On Thu, Mar 01, 2012 at 02:16:57PM +0800, Li Zefan wrote:
> > This can be used to attach metadata to a cgroup, which is required
> > by systemd.

I've not reviewed your patches in detail, but I'm very happy for most
of the shmem xattr code to move out to a library for use elsewhere.

Note that the linux-next mm/shmem.c now includes Jarkko Sakkinen's
shmem_initxattrs() work, maybe you can draw from that too.

> > 
> > As lazy as I am, here just quoted from Lennart for the detailed use
> > cases:
> 
> I much prefer this version but would prefer to deal with this in the
> next merge window as we're already on rc6.  On a cursory look...
> 
> * I'm not sure kmem_ is a good prefix.  It sucks that xattr values are
>   stored in kmem w/o size limit and we probably want to improve on
>   both fronts in the future.  Wouldn't it be better to name them
>   simple_*() and put them in fs/libfs.c?

Yes, simple_*() in fs/libfs.c sounds much better to me than kmem_*().

On the size limit issue, shmem has its own shmem_xattr_validate(),
which rejects XATTR_USER_PREFIX, and the patches leave that unchanged.

I don't know whether the cgroup xattrs need to support USER or not,
and I don't know what permissions would be involved if they do;
but it's an important concern.

Hugh

> 
> * I don't think we want CONFIG option for cgroupfs xattr.  Let's just
>   add a mount option.
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