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>

1 files changed, 5 insertions, 1 deletions

diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt
index 2101e718670d..f1323c6b7ed2 100644
--- 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