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

[Andy Lutomirski <luto@kernel.org>]

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?

Maybe I'm getting far ahead of myself.