fuse: update backing_dev_info congestion state

Set the read and write congestion state if the request queue is close to
blocking, and clear it when it's not.

This prevents unnecessary blocking in readahead and (when writable mmaps are
allowed) writeback.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fuse/dev.c

@@ -224,6 +224,10 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
 			fc->blocked = 0;
 			wake_up_all(&fc->blocked_waitq);
 		}
+		if (fc->num_background == FUSE_CONGESTION_THRESHOLD) {
+			clear_bdi_congested(&fc->bdi, READ);
+			clear_bdi_congested(&fc->bdi, WRITE);
+		}
 		fc->num_background--;
 	}
 	spin_unlock(&fc->lock);
@@ -378,6 +382,10 @@ static void request_send_nowait(struct fuse_conn *fc, struct fuse_req *req)
 		fc->num_background++;
 		if (fc->num_background == FUSE_MAX_BACKGROUND)
 			fc->blocked = 1;
+		if (fc->num_background == FUSE_CONGESTION_THRESHOLD) {
+			set_bdi_congested(&fc->bdi, READ);
+			set_bdi_congested(&fc->bdi, WRITE);
+		}
 
 		queue_request(fc, req);
 		spin_unlock(&fc->lock);

fs/fuse/fuse_i.h

@@ -20,7 +20,10 @@
 #define FUSE_MAX_PAGES_PER_REQ 32
 
 /** Maximum number of outstanding background requests */
-#define FUSE_MAX_BACKGROUND 10
+#define FUSE_MAX_BACKGROUND 12
+
+/** Congestion starts at 75% of maximum */
+#define FUSE_CONGESTION_THRESHOLD (FUSE_MAX_BACKGROUND * 75 / 100)
 
 /** It could be as large as PATH_MAX, but would that have any uses? */
 #define FUSE_NAME_MAX 1024