Re: [PATCH] NFS: Don't skip lookup when holding a delegation

[Olga Kornievskaia <aglo@umich.edu>]

On Thu, Jun 13, 2019 at 11:00 AM J. Bruce Fields <bfields@fieldses.org> wrote:
>
> On Wed, Jun 12, 2019 at 10:45:13AM -0400, Benjamin Coddington wrote:
> > If we skip lookup revalidation while holding a delegation, we might miss
> > that the file has changed directories on the server.
>
> The delegation should prevent the file disappearing from this directory,
> so if I've been following the discussion, the bug was due to overlooking
> the case where the change happened before we got the delegation.  Given
> that history it seems worth calling out that case specifically?
>
> Maybe a comment along the lines of:
>
>                 /*
>                  * Note that the file can't move while we hold a
>                  * delegation.  But this dentry could have been cached
>                  * before we got a delegation.  So it's only safe to
>                  * skip revalidation when the parent directory is
>                  * unchanged:
>                  */
>
> But maybe there's a pithier way to say that.

What is preventing the file from disappearing from the directory while
holding the delegation: is it the server's responsibility to recall
the delegation when it gets a move or is it client's responsibility
not to rely on the cached attributes?

According to this patch it's client's responsibility, in the case, I
find the working " file can't move" confusing as they imply to me that
client can assume file isn't moved (ie, server will prevent it from
happening).

>
> --b.
>
> > The directory's
> > change attribute should still be checked against the dentry's d_time to
> > perform a complete revalidation.
> >
> > Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
> > ---
> >  fs/nfs/dir.c | 10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > index a71d0b42d160..10cc684dc082 100644
> > --- a/fs/nfs/dir.c
> > +++ b/fs/nfs/dir.c
> > @@ -1269,12 +1269,13 @@ nfs_do_lookup_revalidate(struct inode *dir, struct dentry *dentry,
> >               goto out_bad;
> >       }
> >
> > -     if (NFS_PROTO(dir)->have_delegation(inode, FMODE_READ))
> > -             return nfs_lookup_revalidate_delegated(dir, dentry, inode);
> > -
> >       /* Force a full look up iff the parent directory has changed */
> >       if (!(flags & (LOOKUP_EXCL | LOOKUP_REVAL)) &&
> >           nfs_check_verifier(dir, dentry, flags & LOOKUP_RCU)) {
> > +
> > +             if (NFS_PROTO(dir)->have_delegation(inode, FMODE_READ))
> > +                     return nfs_lookup_revalidate_delegated(dir, dentry, inode);
> > +
> >               error = nfs_lookup_verify_inode(inode, flags);
> >               if (error) {
> >                       if (error == -ESTALE)
> > @@ -1707,9 +1708,6 @@ nfs4_do_lookup_revalidate(struct inode *dir, struct dentry *dentry,
> >       if (inode == NULL)
> >               goto full_reval;
> >
> > -     if (NFS_PROTO(dir)->have_delegation(inode, FMODE_READ))
> > -             return nfs_lookup_revalidate_delegated(dir, dentry, inode);
> > -
> >       /* NFS only supports OPEN on regular files */
> >       if (!S_ISREG(inode->i_mode))
> >               goto full_reval;
> > --
> > 2.20.1