Re: [PATCH 0/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2

[Andy Lutomirski]

On Wed, Jun 3, 2026 at 3:43 PM Askar Safin <safinaskar@xxxxxxxxx> wrote:
>
> Andy Lutomirski <luto@xxxxxxxxxxxxxx>:
> > Maybe we should keep an API that does an optimized copy, from one fd
> > to another, that can send from a file to the network with at most ONE
> > cpu-side copy.  Not aiming for zero like sendfile / splice.  Aiming
> > for one.
>
> Yes, this is what my hypothetical future patch will do.
>
> One copy from pagecache to pipe, and then network uses that buffer
> directly.
>
> > But splice_to_socket involves
> > MSG_SPLACE_PAGES, which I think is a part of the mess that you
> > dislike.  And the path where one does copy_splice_read and then
> > splice_to_socket has to be a bit complex because of tee and (I think)
> > because splice_to_socket cannot assume that the incoming data is just
> > ordinary unshared buffers.
>
> My future patch will provide new guarantee: pipe buffers are always
> stable, i. e. they will not be externally-modified.
>
> So hopefully network code will be adjusted to use this guarantee.
>
> But pipe buffers will not be "ordinary unshared buffers".
>
> They still may be shared with other things because of tee(2).
> (But they are still stable! They will not be randomly modified!)
>
> But network code can do "pipe_buf_try_steal" and thus ensure that
> these buffers are not shared with anything else.
>
> So, network code can be modified to use "pipe_buf_try_steal", and you
> will get "ordinary unshared buffers" exactly as you want. This will
> give you in total exactly one copy.
>
> Also: as well as I understand, previously, pipe_buf_try_steal was
> kind of lie. It may return true for buffers created via vmsplice with
> GIFT. (I did not check this, but I think so.) I. e. pipe_buf_try_steal will
> return "true" in this case, but pages are still shared! But, thanks to my
> vmsplice patchset (which is already applied), this is no longer true!
> So now pipe_buf_try_steal is absolutely safe to use!
>
> Finally, we can degrade tee(2) to copy, and hopefully this will
> allow us to always be sure that pipe buffers are not shared with anything.
> This is possible future direction.

I'm a bit nervous that, if I've read the code correctly (a big if),
then iscsi and nvme will still send *shared* buffers via
MSG_SPLICE_PAGES, but that normal user code will not be able to do
this, and that something will bitrot.