bcache: fix wrong return value in bch_debug_init()

in bch_debug_init(), ret is always 0, and the return value is useless,
change it to return 0 if be success after calling debugfs_create_dir(),
else return a non-zero value.
Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/bcache.h

@@ -323,12 +323,6 @@ struct cached_dev {
 	struct bch_ratelimit	writeback_rate;
 	struct delayed_work	writeback_rate_update;
 
-	/*
-	 * Internal to the writeback code, so read_dirty() can keep track of
-	 * where it's at.
-	 */
-	sector_t		last_read;
-
 	/* Limit number of writeback bios in flight */
 	struct semaphore	in_flight;
 	struct task_struct	*writeback_thread;

drivers/md/bcache/debug.c

@@ -251,8 +251,7 @@ void bch_debug_exit(void)
 
 int __init bch_debug_init(struct kobject *kobj)
 {
-	int ret = 0;
-
 	debug = debugfs_create_dir("bcache", NULL);
-	return ret;
+
+	return IS_ERR_OR_NULL(debug);
 }

drivers/md/bcache/writeback.c

@@ -237,7 +237,9 @@ static void read_dirty_submit(struct closure *cl)
 static void read_dirty(struct cached_dev *dc)
 {
 	unsigned delay = 0;
-	struct keybuf_key *w;
+	struct keybuf_key *next, *keys[MAX_WRITEBACKS_IN_PASS], *w;
+	size_t size;
+	int nk, i;
 	struct dirty_io *io;
 	struct closure cl;
 
@@ -248,23 +250,53 @@ static void read_dirty(struct cached_dev *dc)
 	 * mempools.
 	 */
 
-	while (!kthread_should_stop()) {
+	next = bch_keybuf_next(&dc->writeback_keys);
+
+	while (!kthread_should_stop() && next) {
+		size = 0;
+		nk = 0;
+
+		do {
+			BUG_ON(ptr_stale(dc->disk.c, &next->key, 0));
 
-		w = bch_keybuf_next(&dc->writeback_keys);
-		if (!w)
+			/*
+			 * Don't combine too many operations, even if they
+			 * are all small.
+			 */
+			if (nk >= MAX_WRITEBACKS_IN_PASS)
 				break;
 
-		BUG_ON(ptr_stale(dc->disk.c, &w->key, 0));
+			/*
+			 * If the current operation is very large, don't
+			 * further combine operations.
+			 */
+			if (size >= MAX_WRITESIZE_IN_PASS)
+				break;
 
-		if (KEY_START(&w->key) != dc->last_read ||
-		    jiffies_to_msecs(delay) > 50)
-			while (!kthread_should_stop() && delay)
-				delay = schedule_timeout_interruptible(delay);
+			/*
+			 * Operations are only eligible to be combined
+			 * if they are contiguous.
+			 *
+			 * TODO: add a heuristic willing to fire a
+			 * certain amount of non-contiguous IO per pass,
+			 * so that we can benefit from backing device
+			 * command queueing.
+			 */
+			if ((nk != 0) && bkey_cmp(&keys[nk-1]->key,
+						&START_KEY(&next->key)))
+				break;
 
-		dc->last_read	= KEY_OFFSET(&w->key);
+			size += KEY_SIZE(&next->key);
+			keys[nk++] = next;
+		} while ((next = bch_keybuf_next(&dc->writeback_keys)));
 
-		io = kzalloc(sizeof(struct dirty_io) + sizeof(struct bio_vec)
-			     * DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
+		/* Now we have gathered a set of 1..5 keys to write back. */
+		for (i = 0; i < nk; i++) {
+			w = keys[i];
+
+			io = kzalloc(sizeof(struct dirty_io) +
+				     sizeof(struct bio_vec) *
+				     DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
 				     GFP_KERNEL);
 			if (!io)
 				goto err;
@@ -275,7 +307,8 @@ static void read_dirty(struct cached_dev *dc)
 			dirty_init(w);
 			bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
 			io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
-		bio_set_dev(&io->bio, PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
+			bio_set_dev(&io->bio,
+				    PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
 			io->bio.bi_end_io	= read_dirty_endio;
 
 			if (bch_bio_alloc_pages(&io->bio, GFP_KERNEL))
@@ -284,9 +317,20 @@ static void read_dirty(struct cached_dev *dc)
 			trace_bcache_writeback(&w->key);
 
 			down(&dc->in_flight);
+
+			/* We've acquired a semaphore for the maximum
+			 * simultaneous number of writebacks; from here
+			 * everything happens asynchronously.
+			 */
 			closure_call(&io->cl, read_dirty_submit, NULL, &cl);
+		}
 
-		delay = writeback_delay(dc, KEY_SIZE(&w->key));
+		delay = writeback_delay(dc, size);
+
+		while (!kthread_should_stop() && delay) {
+			schedule_timeout_interruptible(delay);
+			delay = writeback_delay(dc, 0);
+		}
 	}
 
 	if (0) {

drivers/md/bcache/writeback.h

@@ -5,6 +5,9 @@
 #define CUTOFF_WRITEBACK	40
 #define CUTOFF_WRITEBACK_SYNC	70
 
+#define MAX_WRITEBACKS_IN_PASS  5
+#define MAX_WRITESIZE_IN_PASS   5000	/* *512b */
+
 static inline uint64_t bcache_dev_sectors_dirty(struct bcache_device *d)
 {
 	uint64_t i, ret = 0;