[PATCH v2] Btrfs: fix find_free_dev_extent() malfunction in case device tree has hole

[Forrest Liu <forrestl@synology.com>]

If device tree has hole, find_free_dev_extent() cannot find available
address properly.

The example below, has one BIG hole in device tree, and can only
allocate just one chunk in a transaction.

    item 9 key (1 DEV_EXTENT 273841913856) itemoff 15811 itemsize 48
        dev extent chunk_tree 3
        chunk objectid 256 chunk offset 272759783424 length 1073741824
    item 10 key (1 DEV_EXTENT 1071632089088) itemoff 15763 itemsize 48
        dev extent chunk_tree 3
        chunk objectid 256 chunk offset 1070549958656 length 1073741824
    item 11 key (1 DEV_EXTENT 1072705830912) itemoff 15715 itemsize 48
            dev extent chunk_tree 3
            chunk objectid 256 chunk offset 1071623700480 length

Signed-off-by: Forrest Liu <forrestl@synology.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
---
changelog:
v2: fix typo key_offset
    replace WARN_ON with WARN_ON_ONCE

 fs/btrfs/volumes.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 50c5a87..da07eca 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1060,6 +1060,7 @@ static int contains_pending_extent(struct btrfs_trans_handle *trans,
 	struct extent_map *em;
 	struct list_head *search_list = &trans->transaction->pending_chunks;
 	int ret = 0;
+	u64 physical_start = *start;
 
 again:
 	list_for_each_entry(em, search_list, list) {
@@ -1070,9 +1071,9 @@ again:
 		for (i = 0; i < map->num_stripes; i++) {
 			if (map->stripes[i].dev != device)
 				continue;
-			if (map->stripes[i].physical >= *start + len ||
+			if (map->stripes[i].physical >= physical_start + len ||
 			    map->stripes[i].physical + em->orig_block_len <=
-			    *start)
+			    physical_start)
 				continue;
 			*start = map->stripes[i].physical +
 				em->orig_block_len;
@@ -1195,8 +1196,14 @@ again:
 			 */
 			if (contains_pending_extent(trans, device,
 						    &search_start,
-						    hole_size))
-				hole_size = 0;
+						    hole_size)) {
+				if (key.offset >= search_start)
+					hole_size = key.offset - search_start;
+				else {
+					WARN_ON_ONCE(1);
+					hole_size = 0;
+				}
+			}
 
 			if (hole_size > max_hole_size) {
 				max_hole_start = search_start;
-- 
1.9.1