Re: [PATCH v1] vfs: allow umount to handle mountpoints without revalidating them

[NeilBrown <neilb@suse.de>]

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On Mon,  1 Jul 2013 09:20:30 -0400 Jeff Layton <jlayton@redhat.com> wrote:

> Christopher reported a regression where he was unable to unmount a NFS
> filesystem where the root had gone stale. The problem is that
> d_revalidate handles the root of the filesystem differently from other
> dentries, but d_weak_revalidate does not. We could simply fix this by
> making d_weak_revalidate return success on IS_ROOT dentries, but there
> are cases where we do want to revalidate the root of the fs.
> 
> A umount is really a special case. We generally aren't interested in
> anything but the dentry and vfsmount that's attached at that point. If
> the inode turns out to be stale we just don't care since the intent is
> to stop using it anyway.
> 
> Try to handle this situation better by treating umount as a special
> case in the lookup code. Have it resolve the parent using normal
> means, and then do a lookup of the final dentry without revalidating
> it. In most cases, the final lookup will come out of the dcache, but
> the case where there's a trailing symlink or !LAST_NORM entry on the
> end complicates things a bit.
> 
> Reported-by: Christopher T Vogan <cvogan@us.ibm.com>
> Signed-off-by: Jeff Layton <jlayton@redhat.com>

Thanks for this Jeff.  It certainly looks credible to me.

There is a lot of code copied from the "user_path_at" path which is a shame,
but probably better that putting in lots of "is this an unmount" tests which
would slow done the common case.

On balance, I like it.

Thanks,
NeilBrown


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