Re: [RFC -next 00/10] Add ZC notifications to splice and sendfile

Joe Damato
2025-03-19T15:32:24Z

On Wed, Mar 19, 2025 at 01:04:48AM -0700, Christoph Hellwig wrote:
> On Wed, Mar 19, 2025 at 12:15:11AM +0000, Joe Damato wrote:
> > One way to fix this is to add zerocopy notifications to sendfile similar
> > to how MSG_ZEROCOPY works with sendmsg. This is possible thanks to the
> > extensive work done by Pavel [1].
>
> What is a "zerocopy notification"

See the docs on MSG_ZEROCOPY [1], but in short when a user app calls
sendmsg and passes MSG_ZEROCOPY a completion notification is added
to the error queue. The user app can poll for these to find out when
the TX has completed and the buffer it passed to the kernel can be
overwritten.

My series provides the same functionality via splice and sendfile2.

[1]: https://www.kernel.org/doc/html/v6.13/networking/msg_zerocopy.html

> and why aren't you simply plugging this into io_uring and generate
> a CQE so that it works like all other asynchronous operations?

I linked to the iouring work that Pavel did in the cover letter.
Please take a look.

That work refactored the internals of how zerocopy completion
notifications are wired up, allowing other pieces of code to use the
same infrastructure and extend it, if needed.

My series is using the same internals that iouring (and others) use
to generate zerocopy completion notifications. Unlike iouring,
though, I don't need a fully customized implementation with a new
user API for harvesting completion events; I can use the existing
mechanism already in the kernel that user apps already use for
sendmsg (the error queue, as explained above and in the
MSG_ZEROCOPY documentation).

Let me know if that answers your question or if you have other
questions.

Thanks,
Joe