Re: [PATCH v3 4/4] io_uring: add support for zone-append

[Jens Axboe]

On 7/6/20 8:10 AM, Matthew Wilcox wrote:
> On Sun, Jul 05, 2020 at 03:12:50PM -0600, Jens Axboe wrote:
>> On 7/5/20 3:09 PM, Matthew Wilcox wrote:
>>> On Sun, Jul 05, 2020 at 03:00:47PM -0600, Jens Axboe wrote:
>>>> On 7/5/20 12:47 PM, Kanchan Joshi wrote:
>>>>> From: Selvakumar S <selvakuma.s1@xxxxxxxxxxx>
>>>>>
>>>>> For zone-append, block-layer will return zone-relative offset via ret2
>>>>> of ki_complete interface. Make changes to collect it, and send to
>>>>> user-space using cqe->flags.
> 
>>> I'm surprised you aren't more upset by the abuse of cqe->flags for the
>>> address.
>>
>> Yeah, it's not great either, but we have less leeway there in terms of
>> how much space is available to pass back extra data.
>>
>>> What do you think to my idea of interpreting the user_data as being a
>>> pointer to somewhere to store the address?  Obviously other things
>>> can be stored after the address in the user_data.
>>
>> I don't like that at all, as all other commands just pass user_data
>> through. This means the application would have to treat this very
>> differently, and potentially not have a way to store any data for
>> locating the original command on the user side.
> 
> I think you misunderstood me.  You seem to have thought I meant
> "use the user_data field to return the address" when I actually meant
> "interpret the user_data field as a pointer to where userspace
> wants the address stored".

It's still somewhat weird to have user_data have special meaning, you're
now having the kernel interpret it while every other command it's just
an opaque that is passed through.

But it could of course work, and the app could embed the necessary
u32/u64 in some other structure that's persistent across IO. If it
doesn't have that, then it'd need to now have one allocated and freed
across the lifetime of the IO.

If we're going that route, it'd be better to define the write such that
you're passing in the necessary information upfront. In syscall terms,
then that'd be something ala:

ssize_t my_append_write(int fd, const struct iovec *iov, int iovcnt,
			off_t *offset, int flags);

where *offset is copied out when the write completes. That removes the
need to abuse user_data, with just providing the storage pointer for the
offset upfront.

-- 
Jens Axboe