Re: [PATCH v2] NFS: Don't skip lookup when holding a delegation

[Trond Myklebust <trondmy@hammerspace.com>]

On Thu, 2020-01-16 at 10:19 -0500, Benjamin Coddington wrote:
> I'd like to bump this one again while we're talking about 
> lookup/revalidation.
> 
> We have folks that hit this problem in the field:
>      - client caches a dentry
>      - file gets moved
>      - server gives out a delegation
>      - client never notices the change because we always skip 
> revalidation
> 
> Is this the wrong place to fix this?  Any other feedback?
> 
> Ben
> 
> On 19 Sep 2019, at 10:49, 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
> > directory's
> > change attribute should still be checked against the dentry's
> > d_time 
> > to
> > perform a complete revalidation.
> > 
> > V2 - Add some commentary as suggested-by J. Bruce Fields.
> > 
> > Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
> > ---
> >  fs/nfs/dir.c | 17 +++++++++++------
> >  1 file changed, 11 insertions(+), 6 deletions(-)
> > 
> > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > index 0adfd8840110..8723e82f5c9d 100644
> > --- a/fs/nfs/dir.c
> > +++ b/fs/nfs/dir.c
> > @@ -1197,12 +1197,20 @@ 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)) {
> > +
> > +		/*
> > +		 * 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:
> > +		 */
> > +		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)
> > @@ -1635,9 +1643,6 @@ 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

We should need to perform this revalidation once, and only once for
that directory, and only if we opened the file using a CLAIM_FH open,
or if we opened it through a different hard linked name (and did not
create this hard link after we got the delegation).

Perhaps we could define a magic value for dentry->d_time that causes us
to skip revalidation if and only if we hold a delegation?

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com