On Tue, 5 Nov 2013 13:39:59 +1100
NeilBrown <neilb@xxxxxxx> wrote:
> On Mon, 4 Nov 2013 20:16:26 -0500 Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> > We had a couple of reports of people that are mounting NFS filesystems,
> > and then bind mounting certain local files onto dentries in that nfs
> > mount (sort of like a poor-man's unionmount).
> >
> > This all works well until the dentry serving as the mountpoint fails
> > d_revalidate. The dentry will end up being invalidated which makes the
> > bind mount unreachable via pathwalk.
> >
> > It doesn't make much sense to me to allow dentries to serve as
> > mountpoints to end up invalidated, so there's no real point in
> > attempting to d_revalidate them at all.
> >
> > Reported-by: Patrick McLean <chutzpah@xxxxxxxxxx>
> > Reported-by: Guang Cheng Li <liguangc@xxxxxxxxxx>
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> > fs/namei.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index caa2805..5b10ad0 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -585,6 +585,9 @@ drop_root_mnt:
> >
> > static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
> > {
> > + /* dentries that serve as mountpoints are always considered valid */
> > + if (d_mountpoint(dentry))
> > + return 1;
> > return dentry->d_op->d_revalidate(dentry, flags);
> > }
> >
>
> Makes sense to me. But what about the parent or grandparent of the mountpoint?
>
> Presumably we don't want to invalidate them either, and I don't think
> d_mountpoint() returns true for those, does it?
>
> NeilBrown
Good point...
I think we're probably saved there (at least on NFS) by the
check_submounts_and_drop() call in nfs_lookup_revalidate.
Hmmm, that said...I did do most of this testing on a slightly older
kernel that doesn't have that call, and I can't reproduce this problem
on a 3.12-ish kernel. I think this is probably already fixed by Miklos'
patches that added that.
Sorry for the noise!
--
Jeff Layton <jlayton@xxxxxxxxxx>
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