netdev - Re: [PATCH net-next v2 17/17] net: Kill MSG_SENDPAGE

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Date: Tue, 20 Jun 2023 08:59:18 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: David Howells <dhowells@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: dhowells@...hat.com, 
 netdev@...r.kernel.org, 
 Alexander Duyck <alexander.duyck@...il.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.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, 
 bpf@...r.kernel.org, 
 dccp@...r.kernel.org, 
 linux-afs@...ts.infradead.org, 
 linux-arm-msm@...r.kernel.org, 
 linux-can@...r.kernel.org, 
 linux-crypto@...r.kernel.org, 
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 linux-x25@...r.kernel.org, 
 mptcp@...ts.linux.dev, 
 rds-devel@....oracle.com, 
 tipc-discussion@...ts.sourceforge.net, 
 virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next v2 17/17] net: Kill MSG_SENDPAGE_NOTLAST

David Howells wrote:
> Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> 
> > Is it intentional to add MSG_MORE here in this patch?
> > 
> > I do see that patch 3 removes this branch:
> 
> Yeah.  I think I may have tcp_bpf a bit wrong with regard to handling
> MSG_MORE.
> 
> How about the attached version of tcp_bpf_push()?
> 
> I wonder if it's save to move the setting of MSG_SENDPAGE_NOPOLICY out of the
> loop as I've done here.  The caller holds the socket lock.
> 
> Also, I'm not sure whether to take account of apply/apply_bytes when setting
> MSG_MORE mid-message, or whether to just go on whether we've reached
> sge->length yet.  (I'm not sure exactly how tcp_bpf works).

I'm not very familiar with it either.

Instead of inferring whether MSG_MORE is safe to set, as below, sufficient to
rely on the caller to pass it when appropriate?

size = min(apply_bytes, sge->length). I doubt that size < apply_bytes is
ever intended.

And instead of this former branch

                if (flags & MSG_SENDPAGE_NOTLAST)
                        msghdr.msg_flags |= MSG_MORE;

update any caller to pass MSG_MORE instead of MSG_SENDPAGE_NOTLAST, if not yet
done so.

> 		msghdr.msg_flags = flags;
> 
> 		/* Determine if we need to set MSG_MORE. */
> 		if (!(msghdr.msg_flags & MSG_MORE)) {
> 			if (apply && size < apply_bytes)
> 				msghdr.msg_flags |= MSG_MORE;
> 			else if (!apply && size < sge->length &&
> 				 msg->sg.start != msg->sg.end)
> 				msghdr.msg_flags |= MSG_MORE;
> 		}