Re: [PATCH v2 1/2] btrfs: Fix lost-data-profile caused by auto removing bg

[Filipe Manana <fdmanana@gmail.com>]

On Wed, Sep 30, 2015 at 12:11 PM, Zhao Lei <zhaolei@cn.fujitsu.com> wrote:
> Reproduce:
>  (In integration-4.3 branch)
>
>  TEST_DEV=(/dev/vdg /dev/vdh)
>  TEST_DIR=/mnt/tmp
>
>  umount "$TEST_DEV" >/dev/null
>  mkfs.btrfs -f -d raid1 "${TEST_DEV[@]}"
>
>  mount -o nospace_cache "$TEST_DEV" "$TEST_DIR"
>  umount "$TEST_DEV"
>
>  mount -o nospace_cache "$TEST_DEV" "$TEST_DIR"
>  btrfs filesystem usage $TEST_DIR
>
> We can see the data chunk changed from raid1 to single:
>  # btrfs filesystem usage $TEST_DIR
>  Data,single: Size:8.00MiB, Used:0.00B
>     /dev/vdg        8.00MiB
>  #
>
> Reason:
>  When a empty filesystem mount with -o nospace_cache, the last
>  data blockgroup will be auto-removed in umount.
>
>  Then if we mount it again, there is no data chunk in the
>  filesystem, so the only available data profile is 0x0, result
>  is all new chunks are created as single type.
>
> Fix:
>  Don't auto-delete last blockgroup for a raid type.
>
> Test:
>  Test by above script, and confirmed the logic by debug output.
>
> Changelog v1->v2:
> 1: Put code of checking block_group->list into
>    semaphore of space_info->groups_sem.
> Noticed-by: Filipe Manana <fdmanana@gmail.com>
>
> Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>

Reviewed-by: Filipe Manana <fdmanana@suse.com>

I would have made the check in the "if" statement below that is
already done while holding a write lock on the semaphore (smaller code
diff), but this is equally correct.

thanks

> ---
>  fs/btrfs/extent-tree.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 79a5bd9..ed9426c 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -10010,8 +10010,18 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
>                 block_group = list_first_entry(&fs_info->unused_bgs,
>                                                struct btrfs_block_group_cache,
>                                                bg_list);
> -               space_info = block_group->space_info;
>                 list_del_init(&block_group->bg_list);
> +
> +               space_info = block_group->space_info;
> +
> +               down_read(&space_info->groups_sem);
> +               if (block_group->list.next == block_group->list.prev) {
> +                       up_read(&space_info->groups_sem);
> +                       btrfs_put_block_group(block_group);
> +                       continue;
> +               }
> +               up_read(&space_info->groups_sem);
> +
>                 if (ret || btrfs_mixed_space_info(space_info)) {
>                         btrfs_put_block_group(block_group);
>                         continue;
> --
> 1.8.5.1
>
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-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."