ceph: don't wait on writeback when there is no more dirty pages

In sync mode, writepages() needs to write all dirty pages. But
it can only write dirty pages associated with the oldest snapc.
To write dirty pages associated with next snapc, it needs to wait
until current writes complete.

If there is no more dirty pages, writepages() should not wait on
writeback. Otherwise, dirty page writeback becomes very slow.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/addr.c

@@ -857,7 +857,7 @@ retry:
 		 * in that range can be associated with newer snapc.
 		 * They are not writeable until we write all dirty pages
 		 * associated with 'snapc' get written */
-		if (index > 0 || wbc->sync_mode != WB_SYNC_NONE)
+		if (index > 0)
 			should_loop = true;
 		dout(" non-head snapc, range whole\n");
 	}
@@ -903,6 +903,10 @@ get_more_pages:
 			if (pgsnapc != snapc) {
 				dout("page snapc %p %lld != oldest %p %lld\n",
 				     pgsnapc, pgsnapc->seq, snapc, snapc->seq);
+				if (!should_loop &&
+				    !ceph_wbc.head_snapc &&
+				    wbc->sync_mode != WB_SYNC_NONE)
+					should_loop = true;
 				unlock_page(page);
 				continue;
 			}