Re: [PATCH] nfsd/blocklayout: accept any minlength

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Oct 09, 2015 at 04:04:38PM -0400, J. Bruce Fields wrote:
> On Fri, Oct 09, 2015 at 01:54:22PM -0400, Trond Myklebust wrote:
> > On Fri, Oct 9, 2015 at 1:45 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> > >
> > > On Fri, Oct 09, 2015 at 07:04:00PM +0200, Christoph Hellwig wrote:
> > > > On Fri, Oct 09, 2015 at 11:28:03AM -0400, J. Bruce Fields wrote:
> > > > > OK, planning to apply for 4.3 just on the assumption that you know what
> > > > > you're doing, but: I don't get it--it looks like the worst that can
> > > > > happen here is we just reuturn LAYOUTUNAVAILABLE to LAYOUTGET.
> > > > > Shouldn't the client then just fall back on normal NFS IO?  Why the
> > > > > hang?
> > > >
> > > > I've just retested with Trond's latest tree and can't reproduce the
> > > > hang anymore.  It used to fence the client due to a lack of response,
> > > > but that might have been a different client bug that has now been fixed.
> > >
> > > OK, makes sense.
> > >
> > > This still looks like a harmless enough change, but is it still stable
> > > and 4.3 material?
> > >
> > > If it affected a released client then it's probably worth it even if
> > > it's really a client bug.  If it's just something you saw once against
> > > an -rc1, I'd rather leave it for 4.4.
> > 
> > It's not a client bug.
> > 
> > The server is supposed to comply with the requirements in table 13 of
> > https://tools.ietf.org/html/rfc5661#section-18.43.3
> > Note that it should also be returning NFS4ERR_BADLAYOUT (or
> > NFS4ERR_LAYOUTTRYLATER if loga_minlength == 0) instead of
> > layoutunavailable if the loga_minlength request cannot be met.
> 
> I had some ideas that layouts were something a server could decline just
> on random whim.  Rereading that section.... OK, looks like I was
> confused, TRYLATER is the closest we come to random whim.

So, updated the changelog as follows, let me know if I've got anything
wrong.

--b.

commit 8c3ad9cb7343
Author: Christoph Hellwig <hch@lst.de>
Date:   Fri Oct 9 15:03:26 2015 +0200

    nfsd/blocklayout: accept any minlength
    
    Recent Linux clients have started to send GETLAYOUT requests with
    minlength less than blocksize.
    
    Servers aren't really allowed to impose this kind of restriction on
    layouts; see RFC 5661 section 18.43.3 for details.
    
    This has been observed to cause indefinite hangs on fsx runs on some
    clients.
    
    Cc: stable@vger.kernel.org
    Signed-off-by: Christoph Hellwig <hch@lst.de>
    Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --git a/fs/nfsd/blocklayout.c b/fs/nfsd/blocklayout.c
index cdefaa331a07..c29d9421bd5e 100644
--- a/fs/nfsd/blocklayout.c
+++ b/fs/nfsd/blocklayout.c
@@ -56,14 +56,6 @@ nfsd4_block_proc_layoutget(struct inode *inode, const struct svc_fh *fhp,
 	u32 device_generation = 0;
 	int error;
 
-	/*
-	 * We do not attempt to support I/O smaller than the fs block size,
-	 * or not aligned to it.
-	 */
-	if (args->lg_minlength < block_size) {
-		dprintk("pnfsd: I/O too small\n");
-		goto out_layoutunavailable;
-	}
 	if (seg->offset & (block_size - 1)) {
 		dprintk("pnfsd: I/O misaligned\n");
 		goto out_layoutunavailable;