linux-kernel - Re: [patch 03/15] cgroup: dont call vfs

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Message-Id: <E1Jt0l8-00066L-NC@pomaz-ex.szeredi.hu>
Date:	Mon, 05 May 2008 15:29:38 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	hch@...radead.org
CC:	miklos@...redi.hu, hch@...radead.org, akpm@...ux-foundation.org,
	viro@...IV.linux.org.uk, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, menage@...gle.com
Subject: Re: [patch 03/15] cgroup: dont call vfs_mkdir

> > > Looks correct but I don't think it's a good idea.  Spreading more logic
> > > into filesystems without a good reason is rarely a good idea.
> > 
> > (Thanks for the review, Christoph)
> > 
> > Agreed completely, but vfs_* aren't for filesystems to call, rather
> > for entities calling _into_ filesystems from the outside.  This is
> > actually a very rare thing, so adding some extra logic for the sake of
> > cleanliness should be OK.
> > 
> > Now it can be argued, that cgroup_clone() is calling into the
> > filesystem from the outside.  But it's not really doing that, rather
> > it's making an internal modification to a specific filesystem,
> > triggered by some external action.
> 
> I don't think that matters. We're not about overly strict layering, and
> especialy this kind where you call into a higher layer to get back into
> the lower one is not harmful at all.  For cgroup it's only a small
> duplication, but e.g. I don't really like all the duplications in the
> reiserfs case.

I think there's some good reasons, other than just to get rid of the
vfs recursion.  I took this change from Jeff Mahoney's patchset.

>  Unless we have a very good reason why the useage of the
> vfs_ function should go away from the filesystem code I don't think
> we want this.

We do have a good reason: r/o bind mounts and AppArmor.  And please
don't tell me, you also think that moving the security hooks to
callers is a good idea ;)  That would actually be a change with a much
larger impact, both in terms of code duplication and of verifying
correctness.

Miklos
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