[PATCH v4 04/11] block: define meta io descriptor

[Anuj Gupta]

On 16/10/24 01:35PM, Keith Busch wrote:
>On Wed, Oct 16, 2024 at 04:59:05PM +0530, Anuj Gupta wrote:
>> +struct uio_meta {
>> +	meta_flags_t	flags;
>> +	u16		app_tag;
>> +	u32		seed;
>> +	struct iov_iter iter;
>> +};
>
>Is the seed used for anything other than the kernel's t10 generation and
>verification? It looks like that's all it is for today, and that part is
>skipped for userspace metadata, so I'm not sure we need it.
>
>I know it's been used for passthrough commands since nvme started
>supporitng it, but I don't see why the driver ever bothered. I think it
>wasn't necessary and we've been carrying it forward ever since.

Not for generation/verfication, but seed is used to remap the ref tag when
submitting metadata on a partition (see blk_integrity_prepare/complete).
For cases like partitioning, MD/DM cloning, where virtual start sector is
different from physical sector remapping is required.

It is skipped for passthrough, but we require it for this series where I/O
can be done on partition too. Christoph [1], Martin [2] also expressed
the need for it in the previous version.

[1] https://lore.kernel.org/linux-block/20240824084430.GG8805@lst.de/
[2] https://lore.kernel.org/linux-block/yq17cc0c9p5.fsf@ca-mkp.ca.oracle.com/