block: Expand a bit documentation about elevator_allow_merge_fn

Explain that two requests can be merged without
elevator_allow_merge_fn() being called.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt
index 2101e71..f1323c6 100644 (file)
--- a/Documentation/block/biodoc.txt
+++ b/Documentation/block/biodoc.txt
@@ -946,7 +946,11 @@ elevator_allow_merge_fn            called whenever the block layer determines
                                request safely. The io scheduler may still
                                want to stop a merge at this point if it
                                results in some sort of conflict internally,
-                               this hook allows it to do that.
+                               this hook allows it to do that. Note however
+                               that two *requests* can still be merged at later
+                               time. Currently the io scheduler has no way to
+                               prevent that. It can only learn about the fact
+                               from elevator_merge_req_fn callback.
 
 elevator_dispatch_fn*          fills the dispatch queue with ready requests.
                                I/O schedulers are free to postpone requests by