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Date: Sun, 02 Apr 2023 11:10:49 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: David Howells <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>
Cc: David Howells <dhowells@...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,
Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: RE: [PATCH v3 15/55] ip, udp: Support MSG_SPLICE_PAGES
David Howells wrote:
> 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
> ---
> net/ipv4/ip_output.c | 102 +++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 99 insertions(+), 3 deletions(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 4e4e308c3230..e2eaba817c1f 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -956,6 +956,79 @@ csum_page(struct page *page, int offset, int copy)
> return csum;
> }
>
> +/*
> + * Allocate a packet for MSG_SPLICE_PAGES.
> + */
> +static int __ip_splice_alloc(struct sock *sk, struct sk_buff **pskb,
> + unsigned int fragheaderlen, unsigned int maxfraglen,
> + unsigned int hh_len)
> +{
> + struct sk_buff *skb_prev = *pskb, *skb;
> + unsigned int fraggap = skb_prev->len - maxfraglen;
> + unsigned int alloclen = fragheaderlen + hh_len + fraggap + 15;
> +
> + skb = sock_wmalloc(sk, alloclen, 1, sk->sk_allocation);
> + if (unlikely(!skb))
> + return -ENOBUFS;
> +
> + /* Fill in the control structures */
> + skb->ip_summed = CHECKSUM_NONE;
> + skb->csum = 0;
> + skb_reserve(skb, hh_len);
> +
> + /* Find where to start putting bytes. */
> + skb_put(skb, fragheaderlen + fraggap);
> + skb_reset_network_header(skb);
> + skb->transport_header = skb->network_header + fragheaderlen;
> + if (fraggap) {
> + skb->csum = skb_copy_and_csum_bits(skb_prev, maxfraglen,
> + skb_transport_header(skb),
> + fraggap);
> + skb_prev->csum = csum_sub(skb_prev->csum, skb->csum);
> + pskb_trim_unique(skb_prev, maxfraglen);
> + }
> +
> + /* Put the packet on the pending queue. */
> + __skb_queue_tail(&sk->sk_write_queue, skb);
> + *pskb = skb;
> + return 0;
> +}
> +
> +/*
> + * 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;
> +}
These functions are derived from and replace ip_append_page.
That can be removed once udp_sendpage is converted?
>
> static int __ip_append_data(struct sock *sk,
> struct flowi4 *fl4,
> struct sk_buff_head *queue,
> @@ -977,7 +1050,7 @@ static int __ip_append_data(struct sock *sk,
> int err;
> int offset = 0;
> bool zc = false;
> - unsigned int maxfraglen, fragheaderlen, maxnonfragsize;
> + unsigned int maxfraglen, fragheaderlen, maxnonfragsize, initial_length;
> int csummode = CHECKSUM_NONE;
> struct rtable *rt = (struct rtable *)cork->dst;
> unsigned int wmem_alloc_delta = 0;
> @@ -1017,6 +1090,7 @@ static int __ip_append_data(struct sock *sk,
> (!exthdrlen || (rt->dst.dev->features & NETIF_F_HW_ESP_TX_CSUM)))
> csummode = CHECKSUM_PARTIAL;
>
> + initial_length = length;
> if ((flags & MSG_ZEROCOPY) && length) {
> struct msghdr *msg = from;
>
> @@ -1047,6 +1121,14 @@ 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 */
> + initial_length = transhdrlen;
I still don't entirely understand what initial_length means.
More importantly, transhdrlen can be zero. If not called for UDP
but for RAW. Or if this is a subsequent call to a packet that is
being held with MSG_MORE.
This works fine for existing use-cases, which go to alloc_new_skb.
Not sure how this case would be different. But the comment alludes
that it does.
> + else
> + flags &= ~MSG_SPLICE_PAGES;
> }
>
> cork->length += length;
> @@ -1074,6 +1156,16 @@ static int __ip_append_data(struct sock *sk,
> unsigned int alloclen, alloc_extra;
> unsigned int pagedlen;
> struct sk_buff *skb_prev;
> +
> + if (unlikely(flags & MSG_SPLICE_PAGES)) {
> + err = __ip_splice_alloc(sk, &skb, fragheaderlen,
> + maxfraglen, hh_len);
> + if (err < 0)
> + goto error;
> + continue;
> + }
> + initial_length = length;
> +
> alloc_new_skb:
> skb_prev = skb;
> if (skb_prev)
> @@ -1085,7 +1177,7 @@ static int __ip_append_data(struct sock *sk,
> * If remaining data exceeds the mtu,
> * we know we need more fragment(s).
> */
> - datalen = length + fraggap;
> + datalen = initial_length + fraggap;
> if (datalen > mtu - fragheaderlen)
> datalen = maxfraglen - fragheaderlen;
> fraglen = datalen + fragheaderlen;
> @@ -1099,7 +1191,7 @@ static int __ip_append_data(struct sock *sk,
> * because we have no idea what fragment will be
> * the last.
> */
> - if (datalen == length + fraggap)
> + if (datalen == initial_length + fraggap)
> alloc_extra += rt->dst.trailer_len;
>
> if ((flags & MSG_MORE) &&
> @@ -1206,6 +1298,10 @@ 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, ©);
> + if (err < 0)
> + goto error;
> } else if (!zc) {
> int i = skb_shinfo(skb)->nr_frags;
>
>
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