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

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On Tue, 2013-07-02 at 11:42 +1000, NeilBrown wrote:
> On Mon,  1 Jul 2013 09:20:30 -0400 Jeff Layton <jlayton@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> 
> 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
> 

(cc'ing Christopher as I mistakenly left him off the original mail. I'll
make sure to cc him on any respins...)

Thanks for looking. Yeah it is a lot of code to handle one case. So,
while this does seem to work, I'm still not 100% sold on this
approach...

I had assumed that we would sometimes want to revalidate IS_ROOT
dentries in other codepaths. Now that I think about it though, I'm
having a hard time coming up with any situations where that's
necessary. We'll never want to invalidate such a dentry, so does that
ever make sense?

If it doesn't, we could just replace this patch with a test for
IS_ROOT(dentry) in nfs_weak_revalidate, and call it a day. I tested a
patch like that earlier and it also worked around the problem.

Also, it bothers me a little that this patch stops revalidating anything
once it hits the last component, even if it's a symlink and we know
we'll have to chase it down. It may make sense to check for d_mountpoint
in some cases and revalidate the dentry if it's true.

Thoughts?
-- 
Jeffrey Layton <jlayton@xxxxxxxxxx>

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