Re: [PATCH v3 15/55] ip, udp: Support MSG_SPLICE_PAGES

David Howells
2023-04-03T22:06:07Z

Willem de Bruijn <willemdebruijn.kernel@gmail.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@redhat.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@redhat.com>
cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: netdev@vger.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;