[PATCH 3/4] ext4: avoid bb_free and bb_fragments inconsistency in mb_free_blocks()

[Baokun Li]

After updating bb_free in mb_free_blocks, it is possible to return without
updating bb_fragments because the block being freed is found to have
already been freed, which leads to inconsistency between bb_free and
bb_fragments.

Since the group may be unlocked in ext4_grp_locked_error(), this can lead
to problems such as dividing by zero when calculating the average fragment
length. Therefore, to ensure consistency, move the update of bb_free to
after the block double-free check.

Fixes: eabe0444df90 ("ext4: speed-up releasing blocks on commit")
CC: stable@xxxxxxxxxxxxxxx # 3.10
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
 fs/ext4/mballoc.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a95fa6e2b0f9..2fbee0f0f5c3 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1892,11 +1892,6 @@ static void mb_free_blocks(struct inode *inode, struct ext4_buddy *e4b,
 	mb_check_buddy(e4b);
 	mb_free_blocks_double(inode, e4b, first, count);
 
-	this_cpu_inc(discard_pa_seq);
-	e4b->bd_info->bb_free += count;
-	if (first < e4b->bd_info->bb_first_free)
-		e4b->bd_info->bb_first_free = first;
-
 	/* access memory sequentially: check left neighbour,
 	 * clear range and then check right neighbour
 	 */
@@ -1922,9 +1917,14 @@ static void mb_free_blocks(struct inode *inode, struct ext4_buddy *e4b,
 				sb, e4b->bd_group,
 				EXT4_GROUP_INFO_BBITMAP_CORRUPT);
 		}
-		goto done;
+		return;
 	}
 
+	this_cpu_inc(discard_pa_seq);
+	e4b->bd_info->bb_free += count;
+	if (first < e4b->bd_info->bb_first_free)
+		e4b->bd_info->bb_first_free = first;
+
 	/* let's maintain fragments counter */
 	if (left_is_free && right_is_free)
 		e4b->bd_info->bb_fragments--;
@@ -1949,7 +1949,6 @@ static void mb_free_blocks(struct inode *inode, struct ext4_buddy *e4b,
 	if (first <= last)
 		mb_buddy_mark_free(e4b, first >> 1, last >> 1);
 
-done:
 	mb_set_largest_free_order(sb, e4b->bd_info);
 	mb_update_avg_fragment_size(sb, e4b->bd_info);
 	mb_check_buddy(e4b);
-- 
2.31.1