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    SubjectRe: [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@szeredi.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@redhat.com>


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