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

Willem de Bruijn
2023-04-04T16:58:37Z

David Howells wrote:
> 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.

If this works, much preferred. Looks great to me.

As said, then __ip_splice_pages() probably no longer needs the
preamble to copy initial header bytes.

> 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;
>
>