drbd: don't BUG_ON, if bio_add_page of a single page to an empty bio fails

Just deal with it more gracefully, if we fail to add even a single page
to an empty bio. We used to BUG_ON() there, but it has been observed in
some Xen deployment, so we need to handle that case more robustly now.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/drivers/block/drbd/drbd_worker.c b/drivers/block/drbd/drbd_worker.c
index ff0eb308ee4a..cfd324b9f95b 100644
--- a/drivers/block/drbd/drbd_worker.c
+++ b/drivers/block/drbd/drbd_worker.c
@@ -369,9 +369,10 @@ static int read_for_csum(struct drbd_conf *mdev, sector_t sector, int size)
         if (drbd_submit_ee(mdev, e, READ, DRBD_FAULT_RS_RD) == 0)
                 return 0;
 
-        /* drbd_submit_ee currently fails for one reason only:
-         * not being able to allocate enough bios.
-         * Is dropping the connection going to help? */
+        /* If it failed because of ENOMEM, retry should help.  If it failed
+         * because bio_add_page failed (probably broken lower level driver),
+         * retry may or may not help.
+         * If it does not, you may need to force disconnect. */
         spin_lock_irq(&mdev->req_lock);
         list_del(&e->w.list);
         spin_unlock_irq(&mdev->req_lock);