linux-kernel - Re: [PATCH net-next v3 01/18] net: Copy slab data for sendmsg(MSG_SPLICE

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Date:   Fri, 23 Jun 2023 11:52:32 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     David Howells <dhowells@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        Alexander Duyck <alexander.duyck@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH net-next v3 01/18] net: Copy slab data for
 sendmsg(MSG_SPLICE_PAGES)

On Fri, 2023-06-23 at 10:08 +0100, David Howells wrote:
> Jakub Kicinski <kuba@...nel.org> wrote:
> 
> > If we can't reach instant agreement --
> > can you strategically separate out the minimal set of changes required
> > to just kill MSG_SENDPAGE_NOTLAST. IMHO it's worth getting that into
> > 6.5.
> 
> Paolo Abeni <pabeni@...hat.com> wrote:
> 
> > Given all the above, and the late stage of the current devel cycle,
> > would you consider slicing down this series to just kill
> > MSG_SENDPAGE_NOTLAST, as Jakub suggested?
> 
> I could do that.
> 
> There is also another alternative.  I could just push the sendpage wrappers up
> the stack into the higher-level callers.  Basically this:
> 
> int udp_sendpage(struct sock *sk, struct page *page, int offset,
> 		 size_t size, int flags)
> {
> 	struct bio_vec bvec;
> 	struct msghdr msg = { .msg_flags = flags | MSG_SPLICE_PAGES };
> 
> 	if (flags & MSG_SENDPAGE_NOTLAST)
> 		msg.msg_flags |= MSG_MORE;
> 
> 	bvec_set_page(&bvec, page, size, offset);
> 	iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, size);
> 	return udp_sendmsg(sk, &msg, size);
> }
> 
> and kill off sendpage and MSG_SENDPAGE_NOTLAST.

I'm unsure I follow the above ?!? I *thought* sendpage could be killed
even without patch 1/18 and 2/18, leaving some patches in this series
unmodified, and mangling those explicitly leveraging 1/18 to use
multiple sendmsg()s with different flags?

I haven't tried to code the above, but my wild guess/hope is that the
delta should be doable - ideally less then the other option.

Introducing slab support should still be possible later, with hopefully
less work.

Cheers,

Paolo