[PATCH v7 06/10] io_uring/rw: add support to send metadata along with read/write

[Kanchan Joshi]

On 11/5/2024 9:53 PM, Keith Busch wrote:
> On Tue, Nov 05, 2024 at 05:00:51PM +0100, Christoph Hellwig wrote:
>> On Tue, Nov 05, 2024 at 09:21:27PM +0530, Kanchan Joshi wrote:
>>> Can add the documentation (if this version is palatable for Jens/Pavel),
>>> but this was discussed in previous iteration:
>>>
>>> 1. Each meta type may have different space requirement in SQE.
>>>
>>> Only for PI, we need so much space that we can't fit that in first SQE.
>>> The SQE128 requirement is only for PI type.
>>> Another different meta type may just fit into the first SQE. For that we
>>> don't have to mandate SQE128.
>>
>> Ok, I'm really confused now.  The way I understood Anuj was that this
>> is NOT about block level metadata, but about other uses of the big SQE.
>>
>> Which version is right?  Or did I just completely misunderstand Anuj?
> 
> Let's not call this "meta_type". Can we use something that has a less
> overloaded meaning, like "sqe_extended_capabilities", or "ecap", or
> something like that.
>   

Right, something like that. We need to change it.
Seems a useful thing is not being seen that way because of its name.

>>> 2. If two meta types are known not to co-exist, they can be kept in the
>>> same place within SQE. Since each meta-type is a flag, we can check what
>>> combinations are valid within io_uring and throw the error in case of
>>> incompatibility.
>>
>> And this sounds like what you refer to is not actually block metadata
>> as in this patchset or nvme, (or weirdly enough integrity in the block
>> layer code).
>>
>>> 3. Previous version was relying on SQE128 flag. If user set the ring
>>> that way, it is assumed that PI information was sent.
>>> This is more explicitly conveyed now - if user passed META_TYPE_PI flag,
>>> it has sent the PI. This comment in the code:
>>>
>>> +       /* if sqe->meta_type is META_TYPE_PI, last 32 bytes are for PI */
>>> +       union {
>>>
>>> If this flag is not passed, parsing of second SQE is skipped, which is
>>> the current behavior as now also one can send regular (non pi)
>>> read/write on SQE128 ring.
>>
>> And while I don't understand how this threads in with the previous
>> statements, this makes sense.  If you only want to send a pointer (+len)
>> to metadata you can use the normal 64-byte SQE.  If you want to send
>> a PI tuple you need SEQ128.  Is that what the various above statements
>> try to express?  If so the right API to me would be to have two flags:
>>
>>   - a flag that a pointer to metadata is passed.  This can work with
>>     a 64-bit SQE.
>>   - another flag that a PI tuple is passed.  This requires a 128-byte
>>     and also the previous flag.
> 
> I don't think anything done so far aligns with what Pavel had in mind.
> Let me try to lay out what I think he's going for. Just bare with me,
> this is just a hypothetical example.

I have the same example in mind.


>    This patch adds a PI extension.
>    Later, let's say write streams needs another extenion.
>    Then key per-IO wants another extention.
>    Then someone else adds wizbang-awesome-feature extention.
> 
> Let's say you have device that can do all 4, or any combination of them.
> Pavel wants a solution that is future proof to such a scenario. So not
> just a single new "meta_type" with its structure, but a list of types in
> no particular order, and their structures.
> 
> That list can exist either in the extended SQE, or in some other user
> address that the kernel will need copy.

That list is the meta_type bit-flags this series creates.

For some future meta_type there can be "META_TYPE_XYZ_INDIRECT" flag and 
that will mean extra-information needs to fetched via copy_from_user.