Re: [POC RFC 0/3] splice(2) support for io_uring

Jens Axboe
2020-01-22T01:56:06Z

On 1/21/20 5:05 PM, Pavel Begunkov wrote:
> It works well for basic cases, but there is still work to be done. E.g.
> it misses @hash_reg_file checks for the second (output) file. Anyway,
> there are some questions I want to discuss:
>
> - why sqe->len is __u32? Splice uses size_t, and I think it's better
> to have something wider (e.g. u64) for fututre use. That's the story
> behind added sqe->splice_len.

IO operations in Linux generally are INT_MAX, so the u32 is plenty big.
That's why I chose it. For this specifically, if you look at splice:

if (unlikely(len > MAX_RW_COUNT))
len = MAX_RW_COUNT;

so anything larger is truncated anyway.

> - it requires 2 fds, and it's painful. Currently file managing is done
> by common path (e.g. io_req_set_file(), __io_req_aux_free()). I'm
> thinking to make each opcode function handle file grabbing/putting
> themself with some helpers, as it's done in the patch for splice's
> out-file.
> 1. Opcode handler knows, whether it have/needs a file, and thus
> doesn't need extra checks done in common path.
> 2. It will be more consistent with splice.
> Objections? Ideas?

Sounds reasonable to me, but always easier to judge in patch form :-)

> - do we need offset pointers with fallback to file->f_pos? Or is it
> enough to have offset value. Jens, I remember you added the first
> option somewhere, could you tell the reasoning?

I recently added support for -1/cur position, which splice also uses. So
you should be fine with that.

--
Jens Axboe