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

[Jens Axboe <axboe@kernel.dk>]

On 7/20/20 7:15 PM, Matthew Wilcox wrote:
>> Also, the completed size should be in res in the first cqe to follow
>> io_uring current interface, no ?. The second cqe would use the res64
>> field to return the written offset. Wasn't that the plan ?
> 
> two cqes for one sqe seems like a bad idea to me.

I have to agree with that, it's counter to everything else. The app will
then have to wait for two CQEs when it issues that one "special" SQE,
which is really iffy. And we'd have to promise that they are adjacent in
the ring. This isn't necessarily a problem right now, but I've been
playing with un-serialized completions and this would then become an
issue. The io_uring interface is clearly defined as "any sqe will either
return an error on submit (if the error is not specific to the sqe
contents), or post a completion event". Not two events, one.

And imho, zoned device append isn't an interesting enough use case to
warrant doing something special. If there was a super strong (and
generic) use case for passing back more information in the cqe then
maybe it would be considered. But it'd have to be a killer application.
If that's not the case, then the use case should work within the
constraints of the existing API.

-- 
Jens Axboe