bcache: Fix dirty_data accounting

Dirty data accounting wasn't quite right - firstly, we were adding the key we're
inserting after it could have merged with another dirty key already in the
btree, and secondly we could sometimes pass the wrong offset to
bcache_dev_sectors_dirty_add() for dirty data we were overwriting - which is
important when tracking dirty data by stripe.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: linux-stable <stable@vger.kernel.org> # >= v3.10

drivers/md/bcache/btree.c

@@ -1742,6 +1742,9 @@ static bool fix_overlapping_extents(struct btree *b,
 		if (bkey_cmp(insert, k) < 0) {
 			bch_cut_front(insert, k);
 		} else {
+			if (bkey_cmp(&START_KEY(insert), &START_KEY(k)) > 0)
+				old_offset = KEY_START(insert);
+
 			if (bkey_written(b, k) &&
 			    bkey_cmp(&START_KEY(insert), &START_KEY(k)) <= 0) {
 				/*
@@ -1803,6 +1806,10 @@ static bool btree_insert_key(struct btree *b, struct btree_op *op,
 		if (fix_overlapping_extents(b, k, &iter, op))
 			return false;
 
+		if (KEY_DIRTY(k))
+			bcache_dev_sectors_dirty_add(b->c, KEY_INODE(k),
+						     KEY_START(k), KEY_SIZE(k));
+
 		while (m != end(i) &&
 		       bkey_cmp(k, &START_KEY(m)) > 0)
 			prev = m, m = bkey_next(m);
@@ -1831,10 +1838,6 @@ static bool btree_insert_key(struct btree *b, struct btree_op *op,
 insert:	shift_keys(b, m, k);
 copy:	bkey_copy(m, k);
 merged:
-	if (KEY_DIRTY(k))
-		bcache_dev_sectors_dirty_add(b->c, KEY_INODE(k),
-					     KEY_START(k), KEY_SIZE(k));
-
 	bch_check_keys(b, "%u for %s", status, op_type(op));
 
 	if (b->level && !KEY_OFFSET(k))