linux-kernel - Re: [POC RFC 0/3] splice(2) support for io

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Date:   Tue, 21 Jan 2020 18:55:59 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:     Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [POC RFC 0/3] splice(2) support for io_uring

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