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

Joe Damato
2025-03-20T18:05:46Z

On Wed, Mar 19, 2025 at 10:50:18PM -0700, Christoph Hellwig wrote:
> On Wed, Mar 19, 2025 at 08:32:19AM -0700, Joe Damato wrote:
> > 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.
>
> Yikes. That's not just an ugly interface, but something entirely
> specific to sockets and incompatible with all other asynchronous I/O
> interfaces.

I don't really know but I would assume it was introduced, as Jens
said, as a work-around long before other completion mechanisms
existed.

> > > 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.
>
> Please write down what matters in the cover letter, including all the
> important tradeoffs.

OK, I will enhance the cover letter for the next submission. I had
originally thought I'd submit something officially, but I think I'll
probably submit another RFC with some of the changes I've made based
on the discussion with Jens.

Namely: dropping sendfile2 completely and plumbing the bits through
for splice. I'll wait a bit to hear what Jens thinks about the
SO_ZEROCOPY thing (basically: if a network socket has that option
set, maybe the existing sendfile can generate error queue
completions without needing a separate system call?).

I agree overall that sendfile2 or sendmsg2 or whatever else could
likely be built differently now that better interfaces and
mechanisms exist in the kernel - but I still think there's room to
improve existing system calls so they can be used safely.