Re: [PATCH 1/3] btrfs: rst: remove encoding field from stripe_extent

Johannes Thumshirn
2024-06-14T09:36:44Z

On 13.06.24 23:23, David Sterba wrote:
> On Tue, Jun 11, 2024 at 04:33:19PM +0000, Johannes Thumshirn wrote:
>> On 11.06.24 16:37, David Sterba wrote:
>>> On Mon, Jun 10, 2024 at 10:40:25AM +0200, Johannes Thumshirn wrote:
>>>> -#define BTRFS_STRIPE_RAID5 5
>>>> -#define BTRFS_STRIPE_RAID6 6
>>>> -#define BTRFS_STRIPE_RAID1C3 7
>>>> -#define BTRFS_STRIPE_RAID1C4 8
>>>> -
>>>> struct btrfs_stripe_extent {
>>>> - __u8 encoding;
>>>> - __u8 reserved[7];
>>>> /* An array of raid strides this stripe is composed of. */
>>>> - struct btrfs_raid_stride strides[];
>>>> + __DECLARE_FLEX_ARRAY(struct btrfs_raid_stride, strides);
>>>
>>> Is there a reason to use the __ underscore macro? I see no difference
>>> between that and DECLARE_FLEX_ARRAY and underscore usually means that
>>> it's special in some way.
>>>
>>
>> Yes, the __ version is for UAPI, like __u8 or __le32 and so on.
>
> I see, though I'd rather keep the on-disk definitions free of wrappers
> that hide the types. We use the __ int types but that's all and quite
> clear what it means.
>
> There already are flexible members (btrfs_leaf, btrfs_node,
> btrfs_inode_extref), using the empty[] syntax. The macro wraps the
> distinction that c++ needs but so far the existing declarations have't
> been problematic. So I'd rather keep the declarations consistent.
>

Yes but all these examples have other members as well. After this patch,
btrfs_stripe_extent is a container for btrfs_raid_stride, and C doesn't
allow a flexmember only struct:

In file included from fs/btrfs/ctree.h:18,
from fs/btrfs/delayed-inode.h:19,
from fs/btrfs/super.c:32:
./include/uapi/linux/btrfs_tree.h:753:34: error: flexible array member
in a struct with no named members
753 | struct btrfs_raid_stride strides[];
| ^~~~~~~