dm: put all polled dm_io instances into a single list

Now that bio_split() isn't used by DM's bio splitting, it is a bit
overkill to link dm_io into an hlist given there is only single dm_io
in the list.

Convert to using a single list for holding all dm_io instances
associated with this bio.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-core.h

@@ -259,7 +259,7 @@ struct dm_io {
 	spinlock_t lock;
 	unsigned long start_time;
 	void *data;
-	struct hlist_node node;
+	struct dm_io *next;
 	struct task_struct *map_task;
 	struct dm_stats_aux stats_aux;
 

drivers/md/dm.c

@@ -1559,7 +1559,7 @@ static bool __process_abnormal_io(struct clone_info *ci, struct dm_target *ti,
 }
 
 /*
- * Reuse ->bi_private as hlist head for storing all dm_io instances
+ * Reuse ->bi_private as dm_io list head for storing all dm_io instances
  * associated with this bio, and this bio's bi_private needs to be
  * stored in dm_io->data before the reuse.
  *
@@ -1567,36 +1567,37 @@ static bool __process_abnormal_io(struct clone_info *ci, struct dm_target *ti,
  * touch it after splitting. Meantime it won't be changed by anyone after
  * bio is submitted. So this reuse is safe.
  */
-static inline struct hlist_head *dm_get_bio_hlist_head(struct bio *bio)
+static inline struct dm_io **dm_poll_list_head(struct bio *bio)
 {
-	return (struct hlist_head *)&bio->bi_private;
+	return (struct dm_io **)&bio->bi_private;
 }
 
 static void dm_queue_poll_io(struct bio *bio, struct dm_io *io)
 {
-	struct hlist_head *head = dm_get_bio_hlist_head(bio);
+	struct dm_io **head = dm_poll_list_head(bio);
 
 	if (!(bio->bi_opf & REQ_DM_POLL_LIST)) {
 		bio->bi_opf |= REQ_DM_POLL_LIST;
 		/*
 		 * Save .bi_private into dm_io, so that we can reuse
-		 * .bi_private as hlist head for storing dm_io list
+		 * .bi_private as dm_io list head for storing dm_io list
 		 */
 		io->data = bio->bi_private;
 
-		INIT_HLIST_HEAD(head);
-
 		/* tell block layer to poll for completion */
 		bio->bi_cookie = ~BLK_QC_T_NONE;
+
+		io->next = NULL;
 	} else {
 		/*
 		 * bio recursed due to split, reuse original poll list,
 		 * and save bio->bi_private too.
 		 */
-		io->data = hlist_entry(head->first, struct dm_io, node)->data;
+		io->data = (*head)->data;
+		io->next = *head;
 	}
 
-	hlist_add_head(&io->node, head);
+	*head = io;
 }
 
 /*
@@ -1685,8 +1686,8 @@ static void dm_split_and_process_bio(struct mapped_device *md,
 	 * Drop the extra reference count for non-POLLED bio, and hold one
 	 * reference for POLLED bio, which will be released in dm_poll_bio
 	 *
-	 * Add every dm_io instance into the hlist_head which is stored in
-	 * bio->bi_private, so that dm_poll_bio can poll them all.
+	 * Add every dm_io instance into the dm_io list head which is stored
+	 * in bio->bi_private, so that dm_poll_bio can poll them all.
 	 */
 	if (error || !ci.submit_as_polled) {
 		/*
@@ -1748,18 +1749,16 @@ static bool dm_poll_dm_io(struct dm_io *io, struct io_comp_batch *iob,
 static int dm_poll_bio(struct bio *bio, struct io_comp_batch *iob,
 		       unsigned int flags)
 {
-	struct hlist_head *head = dm_get_bio_hlist_head(bio);
-	struct hlist_head tmp = HLIST_HEAD_INIT;
-	struct hlist_node *next;
-	struct dm_io *io;
+	struct dm_io **head = dm_poll_list_head(bio);
+	struct dm_io *list = *head;
+	struct dm_io *tmp = NULL;
+	struct dm_io *curr, *next;
 
 	/* Only poll normal bio which was marked as REQ_DM_POLL_LIST */
 	if (!(bio->bi_opf & REQ_DM_POLL_LIST))
 		return 0;
 
-	WARN_ON_ONCE(hlist_empty(head));
-
-	hlist_move_list(head, &tmp);
+	WARN_ON_ONCE(!list);
 
 	/*
 	 * Restore .bi_private before possibly completing dm_io.
@@ -1770,24 +1769,27 @@ static int dm_poll_bio(struct bio *bio, struct io_comp_batch *iob,
 	 * clearing REQ_DM_POLL_LIST here.
 	 */
 	bio->bi_opf &= ~REQ_DM_POLL_LIST;
-	bio->bi_private = hlist_entry(tmp.first, struct dm_io, node)->data;
+	bio->bi_private = list->data;
 
-	hlist_for_each_entry_safe(io, next, &tmp, node) {
-		if (dm_poll_dm_io(io, iob, flags)) {
-			hlist_del_init(&io->node);
+	for (curr = list, next = curr->next; curr; curr = next, next =
+			curr ? curr->next : NULL) {
+		if (dm_poll_dm_io(curr, iob, flags)) {
 			/*
 			 * clone_endio() has already occurred, so no
 			 * error handling is needed here.
 			 */
-			__dm_io_dec_pending(io);
+			__dm_io_dec_pending(curr);
+		} else {
+			curr->next = tmp;
+			tmp = curr;
 		}
 	}
 
 	/* Not done? */
-	if (!hlist_empty(&tmp)) {
+	if (tmp) {
 		bio->bi_opf |= REQ_DM_POLL_LIST;
 		/* Reset bio->bi_private to dm_io list head */
-		hlist_move_list(&tmp, head);
+		*head = tmp;
 		return 0;
 	}
 	return 1;