pNFS: Don't send LAYOUTGET on OPEN for read, if we already have cached data

If we're only opening the file for reading, and the file is empty and/or
we already have cached data, then heuristically optimise away the
LAYOUTGET.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/pnfs.c

@@ -2046,6 +2046,11 @@ static void _lgopen_prepare_attached(struct nfs4_opendata *data,
 	struct nfs4_layoutget *lgp;
 	struct pnfs_layout_hdr *lo;
 
+	/* Heuristic: don't send layoutget if we have cached data */
+	if (rng.iomode == IOMODE_READ &&
+	   (i_size_read(ino) == 0 || ino->i_mapping->nrpages != 0))
+		return;
+
 	lo = _pnfs_grab_empty_layout(ino, ctx);
 	if (!lo)
 		return;