pNFS/filelayout: Fix coalescing test for single DS

When there is a single DS no striping constraints need to be placed on
the IO. When such constraint is applied then buffered reads don't
coalesce to the DS's rsize.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/filelayout/filelayout.c

@@ -783,6 +783,12 @@ filelayout_alloc_lseg(struct pnfs_layout_hdr *layoutid,
 	return &fl->generic_hdr;
 }
 
+static bool
+filelayout_lseg_is_striped(const struct nfs4_filelayout_segment *flseg)
+{
+	return flseg->num_fh > 1;
+}
+
 /*
  * filelayout_pg_test(). Called by nfs_can_coalesce_requests()
  *
@@ -803,6 +809,8 @@ filelayout_pg_test(struct nfs_pageio_descriptor *pgio, struct nfs_page *prev,
 	size = pnfs_generic_pg_test(pgio, prev, req);
 	if (!size)
 		return 0;
+	else if (!filelayout_lseg_is_striped(FILELAYOUT_LSEG(pgio->pg_lseg)))
+		return size;
 
 	/* see if req and prev are in the same stripe */
 	if (prev) {