Re: [PATCH] mountd: Ignore transient and non-fatal filesystem errors in nfsd_export

[Trond Myklebust <trondmy@hammerspace.com>]

On Thu, 2020-09-10 at 13:02 -0400, J. Bruce Fields wrote:
> On Tue, Sep 08, 2020 at 05:19:58PM -0400, trondmy@kernel.org wrote:
> > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> > 
> > If the mount point check in nfsd_export fails due to a transient
> > error,
> > then ignore it to avoid spurious NFSERR_STALE errors being returned
> > by
> > knfsd.
> 
> What sort of transient errors?
> 
> I guess this makes the upcall (and the original rpc) eventually time
> out?

The point here is that is_mountpoint() is making the assumption that
_any_ error from stat() can be used to infer that there is nothing
mounted. In reality, only a subset of errors allow it to make that
assumption (specifically the errors ELOOP, ENAMETOOLONG, ENOENT, and
ENOTDIR).

In our case, we're seeing a problem when the underlying filesystem is a
soft mounted NFSv4.2 client (i.e. we're running our Hammerspace use
case of proxying NFSv4.2 to legacy NFSv3 clients) and that NFS4.2 mount
times out due to a reboot of the underlying server, for instance.

So, yes, in that case we want the upcall to time out instead of poking
knfsd into declaring that the directory is stale.


> 
> --b.
> 
> > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > ---
> >  utils/mountd/cache.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/utils/mountd/cache.c b/utils/mountd/cache.c
> > index 6cba2883026f..93e868341d15 100644
> > --- a/utils/mountd/cache.c
> > +++ b/utils/mountd/cache.c
> > @@ -1411,7 +1411,10 @@ static void nfsd_export(int f)
> >  
> >  		if (mp && !*mp)
> >  			mp = found->m_export.e_path;
> > -		if (mp && !is_mountpoint(mp))
> > +		errno = 0;
> > +		if (mp && !is_mountpoint(mp)) {
> > +			if (errno != 0 && !path_lookup_error(errno))
> > +				goto out;
> >  			/* Exportpoint is not mounted, so tell kernel
> > it is
> >  			 * not available.
> >  			 * This will cause it not to appear in the V4
> > Pseudo-root
> > @@ -1420,9 +1423,12 @@ static void nfsd_export(int f)
> >  			 * And filehandle for this mountpoint from an
> > earlier
> >  			 * mount will block in nfsd.fh lookup.
> >  			 */
> > +			xlog(L_WARNING,
> > +			     "Cannot export path '%s': not a
> > mountpoint",
> > +			     path);
> >  			dump_to_cache(f, buf, sizeof(buf), dom, path,
> >  				      NULL, 60);
> > -		else if (dump_to_cache(f, buf, sizeof(buf), dom, path,
> > +		} else if (dump_to_cache(f, buf, sizeof(buf), dom,
> > path,
> >  					 &found->m_export, 0) < 0) {
> >  			xlog(L_WARNING,
> >  			     "Cannot export %s, possibly unsupported
> > filesystem"
> > -- 
> > 2.26.2
-- 
Trond Myklebust
CTO, Hammerspace Inc
4984 El Camino Real, Suite 208
Los Altos, CA 94022
www.hammer.space