[PATCH] xen-front-pgdir-shbuf: don't record wrong grant handle upon error

[Jan Beulich <jbeulich@xxxxxxxx>]

In order for subsequent unmapping to not mistakenly unmap handle 0,
record a perceived always-invalid one instead.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
---
v2: Use INVALID_GRANT_HANDLE.

--- a/drivers/xen/xen-front-pgdir-shbuf.c
+++ b/drivers/xen/xen-front-pgdir-shbuf.c
@@ -305,11 +305,18 @@ static int backend_map(struct xen_front_
 
        /* Save handles even if error, so we can unmap. */
        for (cur_page = 0; cur_page < buf->num_pages; cur_page++) {
-               buf->backend_map_handles[cur_page] = map_ops[cur_page].handle;
-               if (unlikely(map_ops[cur_page].status != GNTST_okay))
+               if (likely(map_ops[cur_page].status == GNTST_okay)) {
+                       buf->backend_map_handles[cur_page] =
+                               map_ops[cur_page].handle;
+               } else {
+                       buf->backend_map_handles[cur_page] =
+                               INVALID_GRANT_HANDLE;
+                       if (!ret)
+                               ret = -ENXIO;
                        dev_err(&buf->xb_dev->dev,
                                "Failed to map page %d: %d\n",
                                cur_page, map_ops[cur_page].status);
+               }
        }
 
        if (ret) {