linux-kernel - Re: [PATCH 2/2] vfs: force reval on dentry of bind mounted files on FS_REVAL

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Date:	Thu, 3 Dec 2009 10:20:21 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	jamie@...reable.org, pavel@....cz, miklos@...redi.hu,
	viro@...IV.linux.org.uk, duaneg@...da.com, ebiederm@...ssion.com,
	Trond Myklebust <trond.myklebust@....uio.no>
Subject: Re: [PATCH 2/2] vfs: force reval on dentry of bind mounted files on
 FS_REVAL_DOT filesystems

On Thu, 03 Dec 2009 11:58:43 +0100
Miklos Szeredi <miklos@...redi.hu> wrote:

> On Wed,  2 Dec 2009, Jeff Layton wrote:
> > In the case of a bind mounted file, the path walking code will assume
> > that the cached dentry that was bind mounted is valid. This is a problem
> > problem for NFSv4 in a way that's similar to LAST_BIND symlinks.
> > 
> > Fix this by revalidating the dentry if FS_FOLLOW_DOT is set and
> > __follow_mount returns true.
> > 
> > Note that in the non-open codepath, we cannot return an error to the
> > lookup if the revalidation fails. Doing so will leave a bind mount in
> > a state such that we can't unmount it. In that case we'll just have to
> > settle for d_invalidating it (which should mostly turn out to be a
> > d_drop in this case) and returning success.
> 
> The only worry I have is that this adds an extra branch in a very hot
> codepath (do_lookup).  An error can't be returned, as you note, and
> for bind mounted directories d_invalidate() will not succeed: the
> directory is busy, it's referenced by the mount.  So basically the
> only thing this does is working around the NFSv4 issue.  But Trond has
> a proper solution to that, and a temporary solution could be added to
> do_filp_open() rather than burdening do_lookup() with it, no?
> 

(re-adding Trond. I forgot to cc him on this latest set)

Self-NAK on this patch...

That's my main worry too, and sadly it doesn't seem to be unfounded.
This patch adds a lot of extra d_revalidate calls here. I think it's
going to be too expensive to do this.

Unfortunately, adding the code to do_filp_open alone doesn't really
help. The path walking code in there is just for the O_CREAT case. If
we're not creating the file, we call into path_lookup_open which
eventually calls do_lookup.

What we probably need to do is only make it d_revalidate when it looks
like the final dentry is bind mounted. I'm not sure how to tell that
though and even if I could, I'm still leery of adding this to such a
hot codepath.

The only problem I've identified that this fixes is with file bind
mounts and I don't get the impression they're that common. Maybe the
best thing is to just fix the LAST_BIND symlink case for now and wait
for Trond or Al's overhaul of this code.

-- 
Jeff Layton <jlayton@...hat.com>
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