On 2023/12/18 23:14, Jan Kara wrote:

On Mon 18-12-23 22:18:13, Baokun Li wrote:

...

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@vger.kernel.org # 3.10 Signed-off-by: Baokun Li libaokun1@huawei.com

I agree there's no point in updating the allocation info if the bitmap is corrupted. We will not try to allocate (or free) blocks in that group anymore. I'm just a bit unsure about the EXT4_FC_REPLAY state where we don't mark the bitmap as corrupted although some blocks were already marked as freed. So in this case the free space statistics tracking will go permanently wrong. I'm kind of wondering in which case does fast-commit free already freed blocks. Ted, any idea?

						Honza

Some additional information, this judgment was introduced in commit 8016e29f4362 ("ext4: fast commit recovery path") in v5.10-rc1, at which point mb_regenerate_buddy() was called to regenerate the buddy when it was found to be freeing a block that had already been freed, so there was no problem. Until v5.11-rc1 commit 6bd97bf273bd ("ext4: remove redundant mb_regenerate_buddy()") removes the logic to regenerate the buddy, it looks like the free space statistics will remain wrong. If this normal scenario exists, perhaps buddy should be regenerated here?

Thanks, Baokun

...

---

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