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

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Date:	Thu, 03 Dec 2009 03:11:39 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Jeff Layton <jlayton@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, jamie@...reable.org, pavel@....cz,
	viro@...IV.linux.org.uk, duaneg@...da.com
Subject: Re: [PATCH 2/2] vfs: force reval on dentry of bind mounted files on FS_REVAL_DOT filesystems

Miklos Szeredi <miklos@...redi.hu> writes:

> 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. 

Not true.  d_mountpoint is false, so d_invalidate can succeed.

> So basically the
> only thing this does is working around the NFSv4 issue. 

No, this should catch other cases where we have a dentry goes
stale as well, and lets the distributed filesystem handle it.

It is probably worth a benchmark to ease the concerns about the hotpath.
I expect the cpu will predict the branch as unlikely and we won't see
any difference.

Eric
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