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

[Jens Axboe <axboe@kernel.dk>]

On 7/6/20 8:32 AM, Matthew Wilcox wrote:
> On Mon, Jul 06, 2020 at 08:27:17AM -0600, Jens Axboe wrote:
>> 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@samsung.com>
>>>>>>>
>>>>>>> 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.
> 
> That works for me!  In io_uring terms, would you like to see that done
> as adding:
> 
>         union {
>                 __u64   off;    /* offset into file */
> +		__u64   *offp;	/* appending writes */
>                 __u64   addr2;
>         };
> 

Either that, or just use addr2 for it directly. I consider the appending
writes a marginal enough use case that it doesn't really warrant adding
a specially named field for that.

-- 
Jens Axboe