Re: copy on write for splice() from file to pipe?

Jens Axboe
2023-02-10T21:52:35Z

On 2/10/23 2:27 PM, Jens Axboe wrote:
> On 2/10/23 2:14?PM, Andy Lutomirski wrote:
>> On Fri, Feb 10, 2023 at 12:50 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> On 2/10/23 1:44?PM, Linus Torvalds wrote:
>>>> On Fri, Feb 10, 2023 at 12:39 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>
>>>>> Right, I'm referencing doing zerocopy data sends with io_uring, using
>>>>> IORING_OP_SEND_ZC. This isn't from a file, it's from a memory location,
>>>>> but the important bit here is the split notifications and how you
>>>>> could wire up a OP_SENDFILE similarly to what Andy described.
>>>>
>>>> Sure, I think it's much more reasonable with io_uring than with splice itself.
>>>>
>>>> So I was mainly just reacting to the "strict-splice" thing where Andy
>>>> was talking about tracking the page refcounts. I don't think anything
>>>> like that can be done at a splice() level, but higher levels that
>>>> actually know about the whole IO might be able to do something like
>>>> that.
>>>>
>>>> Maybe we're just talking past each other.
>>>
>>> Maybe slightly, as I was not really intending to comment on the strict
>>> splice thing. But yeah I agree on splice, it would not be trivial to do
>>> there. At least with io_uring we have the communication channel we need.
>>> And tracking page refcounts seems iffy and fraught with potential
>>> issues.
>>>
>>
>> Hmm.
>>
>> Are there any real-world use cases for zero-copy splice() that
>> actually depend on splicing from a file to a pipe and then later from
>> the pipe to a socket (or file or whatever)? Or would everything
>> important be covered by a potential new io_uring operation that copies
>> from one fd directly to another fd?
>
> I think it makes sense. As Linus has referenced, the sex appeal of
> splice is the fact that it is dealing with pipes, and you can access
> these internal buffers through other means. But that is probably largely
> just something that is sexy design wise, nothing that _really_ matters
> in practice. And the pipes do get in the way, for example I had to add
> pipe resizing fcntl helpers to bump the size. If you're doing a plain
> sendfile, the pipes just kind of get in the way too imho.
>
> Another upside (from the io_uring) perspective is that splice isn't very
> efficient through io_uring, as it requires offload to io-wq. This could
> obviously be solved by some refactoring in terms of non-blocking, but it
> hasn't really been that relevant (and nobody has complained about it). A
> new sendfile op would nicely get around that too as it could be designed
> with async in nature, rather than the classic sync syscall model that
> splice follows.

Speaking of splice/io_uring, Ming posted this today:

https://lore.kernel.org/io-uring/20230210153212.733006-1-ming.lei@redhat.com/

--
Jens Axboe