Re: [nfs-ganesha]{PATCH] FSAL_CEPH: do an inode lookup vs. MDS when the Inode is not in cache

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2017-11-10 at 10:31 +0800, Yan, Zheng wrote:
> On Fri, Nov 10, 2017 at 12:03 AM, Jeff Layton <jlayton@kernel.org> wrote:
> > From: Jeff Layton <jlayton@redhat.com>
> > 
> > FSAL_CEPH just does a ceph_ll_get_inode when asked to create a
> > filehandle today, which only does a query of the local Ceph client
> > cache. This means, it can only find Inodes that were previously found
> > via name lookup.
> > 
> > When ganesha is restarted, we have a blank-slate Client, and the Inode
> > will not be in cache. That ends up with the NFS client getting back an
> > ESTALE.
> > 
> > We must be able to find Inodes that may not be in the cache. When
> > ceph_ll_get_inode can't find an Inode, try a lookup vs. the MDS for it.
> > 
> > The only problem is that we currently have no way to look up a snapped
> > inode from scratch. I think we'll probably need to add that ability to
> > libcephfs, but for now the best we can do is just bail out with an
> > ESTALE in that case.
> > 
> > While we're in there, remove the comment about ceph_ll_connectable_m.
> > There is no such function in libcephfs and never has been. The ds
> > code does seem to refer to such a call, but it's not clear to me
> > what it's supposed to do.
> > 
> > Change-Id: I2d0e362ef8f28ba78575b60f3fb2890096d98fc6
> > Signed-off-by: Jeff Layton <jlayton@redhat.com>
> > ---
> > 
> > I sent this up through git-review to ganesha's gerrit, but I'd
> > like some cephfs folks to have a look too. This patch is basically
> > a workaround for the FSAL_CEPH code for now.
> > 
> > Do we need to worry about inodes that are not CEPH_NOSNAP? It
> > seems like we might need for libcephfs to grow an interface that
> > can look up snapped inodes as well?
> > 
> > Either that, or teach ceph_ll_get_inode how to look up inodes
> > that are not in its cache. Would that be a better fix here?
> > 
> >  src/FSAL/FSAL_CEPH/export.c | 19 +++++++++++++++----
> >  1 file changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/src/FSAL/FSAL_CEPH/export.c b/src/FSAL/FSAL_CEPH/export.c
> > index 3de25fdfaf41..a31e3bb3ebef 100644
> > --- a/src/FSAL/FSAL_CEPH/export.c
> > +++ b/src/FSAL/FSAL_CEPH/export.c
> > @@ -234,12 +234,23 @@ static fsal_status_t create_handle(struct fsal_export *export_pub,
> >                 return status;
> >         }
> > 
> > +       /* Check our local cache first */
> >         i = ceph_ll_get_inode(export->cmount, *vi);
> > -       if (!i)
> > -               return ceph2fsal_error(-ESTALE);
> > +       if (!i) {
> > +               /*
> > +                * Try the slow way, may not be in cache now.
> > +                *
> > +                * Currently, there is no interface for looking up a snapped
> > +                * inode, so we just bail here in that case.
> > +                */
> > +               if (vi->snapid.val != CEPH_NOSNAP)
> > +                       return ceph2fsal_error(-ESTALE);
> > +
> > +               rc = ceph_ll_lookup_inode(export->cmount, vi->ino, &i);
> > +               if (rc)
> > +                       return ceph2fsal_error(rc);
> > +       }
> > 
> > -       /* The ceph_ll_connectable_m should have populated libceph's
> > -          cache with all this anyway */
> >         rc = fsal_ceph_ll_getattr(export->cmount, i, &stx,
> >                 attrs_out ? CEPH_STATX_ATTR_MASK : CEPH_STATX_HANDLE_MASK,
> >                 op_ctx->creds);
> > --
> > 2.13.6
> > 
> 
> Looks good to me,
> 
> Tested-by: "Yan, Zheng" <zyan@redhat.com>
> 

Thanks. I still wonder whether this is a sufficient fix.

ceph_ll_lookup_inode does not take the snapid into account. Do we need a
version of ceph_ll_lookup_inode that can find snapshotted inodes after
ganesha has restarted?

It does seem like we could allow a nfs client do work in a snapshotted
directory, and then have ganesha restart. That client may be very
confused when the server comes back and all of the existing filehandles
go stale.
-- 
Jeff Layton <jlayton@kernel.org>