bcache: add code comments for journal_read_bucket()

This patch adds more code comments in journal_read_bucket(), this is an
effort to make the code to be more understandable.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/journal.c

@@ -100,6 +100,20 @@ reread:		left = ca->sb.bucket_size - offset;
 
 			blocks = set_blocks(j, block_bytes(ca->set));
 
+			/*
+			 * Nodes in 'list' are in linear increasing order of
+			 * i->j.seq, the node on head has the smallest (oldest)
+			 * journal seq, the node on tail has the biggest
+			 * (latest) journal seq.
+			 */
+
+			/*
+			 * Check from the oldest jset for last_seq. If
+			 * i->j.seq < j->last_seq, it means the oldest jset
+			 * in list is expired and useless, remove it from
+			 * this list. Otherwise, j is a condidate jset for
+			 * further following checks.
+			 */
 			while (!list_empty(list)) {
 				i = list_first_entry(list,
 					struct journal_replay, list);
@@ -109,13 +123,22 @@ reread:		left = ca->sb.bucket_size - offset;
 				kfree(i);
 			}
 
+			/* iterate list in reverse order (from latest jset) */
 			list_for_each_entry_reverse(i, list, list) {
 				if (j->seq == i->j.seq)
 					goto next_set;
 
+				/*
+				 * if j->seq is less than any i->j.last_seq
+				 * in list, j is an expired and useless jset.
+				 */
 				if (j->seq < i->j.last_seq)
 					goto next_set;
 
+				/*
+				 * 'where' points to first jset in list which
+				 * is elder then j.
+				 */
 				if (j->seq > i->j.seq) {
 					where = &i->list;
 					goto add;
@@ -129,6 +152,7 @@ reread:		left = ca->sb.bucket_size - offset;
 			if (!i)
 				return -ENOMEM;
 			memcpy(&i->j, j, bytes);
+			/* Add to the location after 'where' points to */
 			list_add(&i->list, where);
 			ret = 1;