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

["bfields@fieldses.org" <bfields@fieldses.org>]

On Mon, Jun 14, 2021 at 07:53:52PM +0000, Trond Myklebust wrote:
> 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.

Or 1, actually (FILE_LOCK_DEFERRED).

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

I can live with that, but I'm still curious what exactly you're worried
about.

--b.