Re: [PATCH 3/3] nfs: don't allow reexport reclaims

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2021-06-14 at 15:34 -0400, J. Bruce Fields wrote:
> On Mon, Jun 14, 2021 at 02:56:55PM +0000, Trond Myklebust wrote:
> > On Mon, 2021-06-14 at 10:48 -0400, J. Bruce Fields wrote:
> > > From: "J. Bruce Fields" <bfields@redhat.com>
> > > 
> > > In the reexport case, nfsd is currently passing along locks with
> > > the
> > > reclaim bit set.  The client sends a new lock request, which is
> > > granted
> > > if there's currently no conflict--even if it's possible a
> > > conflicting
> > > lock could have been briefly held in the interim.
> > > 
> > > We don't currently have any way to safely grant reclaim, so for
> > > now
> > > let's just deny them all.
> > > 
> > > I'm doing this by passing the reclaim bit to nfs and letting it
> > > fail
> > > the
> > > call, with the idea that eventually the client might be able to
> > > do
> > > something more forgiving here.
> > > 
> > > Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> > > ---
> > >  fs/nfs/file.c       | 3 +++
> > >  fs/nfsd/nfs4state.c | 3 +++
> > >  fs/nfsd/nfsproc.c   | 1 +
> > >  include/linux/fs.h  | 1 +
> > >  4 files changed, 8 insertions(+)
> > > 
> > > diff --git a/fs/nfs/file.c b/fs/nfs/file.c
> > > index 1fef107961bc..35a29b440e3e 100644
> > > --- a/fs/nfs/file.c
> > > +++ b/fs/nfs/file.c
> > > @@ -806,6 +806,9 @@ int nfs_lock(struct file *filp, int cmd,
> > > struct
> > > file_lock *fl)
> > >  
> > >         nfs_inc_stats(inode, NFSIOS_VFSLOCK);
> > >  
> > > +       if (fl->fl_flags & FL_RECLAIM)
> > > +               return -NFSERR_NO_GRACE;
> > 
> > NACK. nfs_lock() is required to return a POSIX error. I know that
> > right
> > now, nfsd is the only thing setting FL_RECLAIM, but we can't
> > guarantee
> > that will always be the case.
> 
> Setting FL_RECLAIM tells the filesystem that you're prepared to
> handle
> NFSERR_NO_GRACE.  I'm not seeing the risk.

You are using a function that is exposed to the VFS. On error, that
function is expected to return a value that is a Linux error between -1
and -4095.

I suggest adding an error value ENOGRACE to include/linux/errno.h.

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