[PATCH 03/20] ext4: fix double brelse() the buffer of the extents path

[libaokun]

From: Baokun Li <libaokun1@xxxxxxxxxx>

In ext4_ext_try_to_merge_up(), set path[1].p_bh to NULL after it has been
released, otherwise it may be released twice.

An example of what triggers this is as follows:

  split2    map    split1
|--------|-------|--------|

ext4_ext_map_blocks
 ext4_ext_handle_unwritten_extents
  ext4_split_convert_extents
   // path->p_depth == 0
   ext4_split_extent
     // 1. do split1
     ext4_split_extent_at
       ext4_ext_insert_extent
         ext4_ext_create_new_leaf
           ext4_ext_grow_indepth
             le16_add_cpu(&neh->eh_depth, 1)
           ext4_find_extent
             path->p_depth = 1
         ext4_ext_try_to_merge
           ext4_ext_try_to_merge_up
             path->p_depth = 0
             brelse(path[1].p_bh)  ---> not set to NULL here
     // 2. update path
     ext4_find_extent
     // 3. do split2
     ext4_split_extent_at
       ext4_ext_insert_extent
         ext4_ext_create_new_leaf
           ext4_ext_grow_indepth
             le16_add_cpu(&neh->eh_depth, 1)
           ext4_find_extent
             path[0].p_bh = NULL;
             path->p_depth = 1
             read_extent_tree_block  ---> return err
             // path[1].p_bh is still the old value
 ext4_free_ext_path
  ext4_ext_drop_refs
   // path->p_depth == 1
   brelse(path[1].p_bh)  ---> brelse a buffer twice

Finally got the following WARRNING when removing the buffer from lru:

============================================
VFS: brelse: Trying to free free buffer
WARNING: CPU: 2 PID: 72 at fs/buffer.c:1241 __brelse+0x58/0x90
CPU: 2 PID: 72 Comm: kworker/u19:1 Not tainted 6.9.0-dirty #716
RIP: 0010:__brelse+0x58/0x90
Call Trace:
 <TASK>
 __find_get_block+0x6e7/0x810
 bdev_getblk+0x2b/0x480
 __ext4_get_inode_loc+0x48a/0x1240
 ext4_get_inode_loc+0xb2/0x150
 ext4_reserve_inode_write+0xb7/0x230
 __ext4_mark_inode_dirty+0x144/0x6a0
 ext4_ext_insert_extent+0x9c8/0x3230
 ext4_ext_map_blocks+0xf45/0x2dc0
 ext4_map_blocks+0x724/0x1700
 ext4_do_writepages+0x12d6/0x2a70
[...]
============================================

Fixes: ecb94f5fdf4b ("ext4: collapse a single extent tree block into the inode if possible")
Cc: stable@xxxxxxxxxx
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
 fs/ext4/extents.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 4d589d34b30e..657baf3991c1 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1888,6 +1888,7 @@ static void ext4_ext_try_to_merge_up(handle_t *handle,
 	path[0].p_hdr->eh_max = cpu_to_le16(max_root);
 
 	brelse(path[1].p_bh);
+	path[1].p_bh = NULL;
 	ext4_free_blocks(handle, inode, NULL, blk, 1,
 			 EXT4_FREE_BLOCKS_METADATA | EXT4_FREE_BLOCKS_FORGET);
 }
-- 
2.39.2