Re: [PATCH] btrfs: update superblock's device bytes_used when dropping chunk

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2025/5/29 19:07, Mark Harmstone 写道:
> Each superblock contains a copy of the device item for that device. In a
> transaction which drops a chunk but doesn't create any new ones, we were
> correctly updating the device item in the chunk tree but not copying
> over the new bytes_used value to the superblock.

Oh! This explains why the image in 
fsck/020/keyed_data_ref_with_shared_leaf.img has the mismatched result.

It's not an older kernel (almost 4 years after another related kernel 
fix), but really some bug in the kernel.

> 
> This can be seen by doing the following:
> 
>   # cd
>   # dd if=/dev/zero of=test bs=4096 count=2621440
>   # mkfs.btrfs test
>   # mount test /root/temp
> 
>   # cd /root/temp
>   # for i in {00..10}; do dd if=/dev/zero of=$i bs=4096 count=32768; done
>   # sync
>   # rm *
>   # sync
>   # btrfs balance start -dusage=0 .
>   # sync
> 
>   # cd
>   # umount /root/temp
>   # btrfs check test
> 
> (For btrfs-check to detect this, you will also need my patch at
> https://github.com/kdave/btrfs-progs/pull/991.)
> 
> Change btrfs_remove_dev_extents() so that it adds the devices to the
> post_commit_list if they're not there already. This causes
> btrfs_commit_device_sizes() to be called, which updates the bytes_used
> value in the superblock.
> 
> Signed-off-by: Mark Harmstone <maharmstone@fb.com>

Looks good to me.

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu> ---
>   fs/btrfs/volumes.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index e59aa0b5c4f3..ee886dc08d15 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3272,6 +3272,12 @@ int btrfs_remove_dev_extents(struct btrfs_trans_handle *trans,
>   					device->bytes_used - dev_extent_len);
>   			atomic64_add(dev_extent_len, &fs_info->free_chunk_space);
>   			btrfs_clear_space_info_full(fs_info);
> +
> +			if (list_empty(&device->post_commit_list)) {
> +				list_add_tail(&device->post_commit_list,
> +					      &trans->transaction->dev_update_list);
> +			}
> +
>   			mutex_unlock(&fs_info->chunk_mutex);
>   		}
>   	}