netdev - Re: [PATCH v3 15/55] ip, udp: Support MSG_SPLICE

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Date:   Mon, 03 Apr 2023 23:04:56 +0100
From:   David Howells <dhowells@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Chuck Lever III <chuck.lever@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        netdev@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 15/55] ip, udp: Support MSG_SPLICE_PAGES

Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:

> The code already has to avoid allocation in the MSG_ZEROCOPY case. I
> added alloc_len and paged_len for that purpose.
> 
> Only the transhdrlen will be copied with getfrag due to
> 
>     copy = datalen - transhdrlen - fraggap - pagedlen
> 
> On next iteration in the loop, when remaining data fits in the skb,
> there are three cases. The first is skipped due to !NETIF_F_SG. The
> other two are either copy to page frags or zerocopy page frags.
> 
> I think your code should be able to fit in. Maybe easier if it could
> reuse the existing alloc_new_skb code to copy the transport header, as
> MSG_ZEROCOPY does, rather than adding a new __ip_splice_alloc branch
> that short-circuits that. Then __ip_splice_pages also does not need
> code to copy the initial header. But this is trickier. It's fine to
> leave as is.
> 
> Since your code currently does call continue before executing the rest
> of that branch, no need to modify any code there? Notably replacing
> length with initial_length, which itself is initialized to length in
> all cases expect for MSG_SPLICE_PAGES.

Okay.  How about the attached?  This seems to work.  Just setting "paged" to
true seems to do the right thing in __ip_append_data() when allocating /
setting up the skbuff, and then __ip_splice_pages() is called to add the
pages.

David
---
commit 9ac72c83407c8aef4be0c84513ec27bac9cfbcaa
Author: David Howells <dhowells@...hat.com>
Date:   Thu Mar 9 14:27:29 2023 +0000

    ip, udp: Support MSG_SPLICE_PAGES
    
    Make IP/UDP sendmsg() support MSG_SPLICE_PAGES.  This causes pages to be
    spliced from the source iterator.
    
    This allows ->sendpage() to be replaced by something that can handle
    multiple multipage folios in a single transaction.
    
    Signed-off-by: David Howells <dhowells@...hat.com>
    cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>
    cc: "David S. Miller" <davem@...emloft.net>
    cc: Eric Dumazet <edumazet@...gle.com>
    cc: Jakub Kicinski <kuba@...nel.org>
    cc: Paolo Abeni <pabeni@...hat.com>
    cc: Jens Axboe <axboe@...nel.dk>
    cc: Matthew Wilcox <willy@...radead.org>
    cc: netdev@...r.kernel.org

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 6109a86a8a4b..fe2e48874191 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -956,6 +956,41 @@ csum_page(struct page *page, int offset, int copy)
 	return csum;
 }
 
+/*
+ * Add (or copy) data pages for MSG_SPLICE_PAGES.
+ */
+static int __ip_splice_pages(struct sock *sk, struct sk_buff *skb,
+			     void *from, int *pcopy)
+{
+	struct msghdr *msg = from;
+	struct page *page = NULL, **pages = &page;
+	ssize_t copy = *pcopy;
+	size_t off;
+	int err;
+
+	copy = iov_iter_extract_pages(&msg->msg_iter, &pages, copy, 1, 0, &off);
+	if (copy <= 0)
+		return copy ?: -EIO;
+
+	err = skb_append_pagefrags(skb, page, off, copy);
+	if (err < 0) {
+		iov_iter_revert(&msg->msg_iter, copy);
+		return err;
+	}
+
+	if (skb->ip_summed == CHECKSUM_NONE) {
+		__wsum csum;
+
+		csum = csum_page(page, off, copy);
+		skb->csum = csum_block_add(skb->csum, csum, skb->len);
+	}
+
+	skb_len_add(skb, copy);
+	refcount_add(copy, &sk->sk_wmem_alloc);
+	*pcopy = copy;
+	return 0;
+}
+
 static int __ip_append_data(struct sock *sk,
 			    struct flowi4 *fl4,
 			    struct sk_buff_head *queue,
@@ -1047,6 +1082,15 @@ static int __ip_append_data(struct sock *sk,
 				skb_zcopy_set(skb, uarg, &extra_uref);
 			}
 		}
+	} else if ((flags & MSG_SPLICE_PAGES) && length) {
+		if (inet->hdrincl)
+			return -EPERM;
+		if (rt->dst.dev->features & NETIF_F_SG) {
+			/* We need an empty buffer to attach stuff to */
+			paged = true;
+		} else {
+			flags &= ~MSG_SPLICE_PAGES;
+		}
 	}
 
 	cork->length += length;
@@ -1206,6 +1250,10 @@ static int __ip_append_data(struct sock *sk,
 				err = -EFAULT;
 				goto error;
 			}
+		} else if (flags & MSG_SPLICE_PAGES) {
+			err = __ip_splice_pages(sk, skb, from, &copy);
+			if (err < 0)
+				goto error;
 		} else if (!zc) {
 			int i = skb_shinfo(skb)->nr_frags;