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

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Date:   Mon, 19 Jun 2023 13:05:27 +0100
From:   David Howells <dhowells@...hat.com>
To:     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, linux-doc@...r.kernel.org,
        linux-hams@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-sctp@...r.kernel.org,
        linux-wpan@...r.kernel.org, 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

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).

David
---

static int tcp_bpf_push(struct sock *sk, struct sk_msg *msg, u32 apply_bytes,
			int flags, bool uncharge)
{
	bool apply = apply_bytes;
	struct scatterlist *sge;
	struct page *page;
	int size, ret = 0;
	u32 off;

	flags |= MSG_SPLICE_PAGES;
	if (tls_sw_has_ctx_tx(sk))
		msghdr.msg_flags |= MSG_SENDPAGE_NOPOLICY;

	while (1) {
		struct msghdr msghdr = {};
		struct bio_vec bvec;

		sge = sk_msg_elem(msg, msg->sg.start);
		size = (apply && apply_bytes < sge->length) ?
			apply_bytes : sge->length;
		off  = sge->offset;
		page = sg_page(sge);

		tcp_rate_check_app_limited(sk);
retry:
		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;
		}

		bvec_set_page(&bvec, page, size, off);
		iov_iter_bvec(&msghdr.msg_iter, ITER_SOURCE, &bvec, 1, size);
		ret = tcp_sendmsg_locked(sk, &msghdr, size);
		if (ret <= 0)
			return ret;

		if (apply)
			apply_bytes -= ret;
		msg->sg.size -= ret;
		sge->offset += ret;
		sge->length -= ret;
		if (uncharge)
			sk_mem_uncharge(sk, ret);
		if (ret != size) {
			size -= ret;
			off  += ret;
			goto retry;
		}
		if (!sge->length) {
			put_page(page);
			sk_msg_iter_next(msg, start);
			sg_init_table(sge, 1);
			if (msg->sg.start == msg->sg.end)
				break;
		}
		if (apply && !apply_bytes)
			break;
	}

	return 0;
}