Re: [RFC PATCH v2 16/48] ip, udp: Support MSG_SPLICE_PAGES

Willem de Bruijn
2023-03-30T17:55:26Z

David Howells wrote:
> Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:
>
> > > + unsigned int maxfraglen, fragheaderlen, maxnonfragsize, xlength;
> >
> > Does x here stand for anything?
>
> Yeah... "bad naming". How about if I call it initial_length? I'm trying to
> avoid allocating bufferage for the data.

That's more informative, thanks. Let me not bikeshed this further for now.

> > This does add a lot of code to two functions that are already
> > unwieldy. It may be unavoidable, but it if can use helpers, that would
> > be preferable.
>
> Something like the attached? (This is on top of patches 16-17, but I would
> need to fold it in)

Yes exactly. I wasn't sure whether the inner loops required access to
too many function scope variables to pull this off. But seems like it
is doable. Great.

>
> David
> ---
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index b38dbb2f9c3f..019ed9bb6745 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -956,6 +956,96 @@ 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, size_t *pcopy)
> +{
> + struct msghdr *msg = from;
> + struct page *page = NULL, **pages = &page;
> + ssize_t copy = *pcopy;
> + size_t off;
> + bool put = false;
> + int err;
> +
> + copy = iov_iter_extract_pages(&msg->msg_iter, &pages, copy, 1, 0, &off);
> + if (copy <= 0)
> + return copy ?: -EIO;
> +
> + if (!sendpage_ok(page)) {
> + const void *p = kmap_local_page(page);
> + void *q;
> +
> + q = page_frag_memdup(NULL, p + off, copy,
> + sk->sk_allocation, ULONG_MAX);
> + kunmap_local(p);
> + if (!q)
> + return -ENOMEM;
> + page = virt_to_page(q);
> + off = offset_in_page(q);
> + put = true;
> + }
> +
> + err = skb_append_pagefrags(skb, page, off, copy);
> + if (put)
> + put_page(page);
> + 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,
> @@ -977,7 +1067,7 @@ static int __ip_append_data(struct sock *sk,
> int err;
> int offset = 0;
> bool zc = false;
> - unsigned int maxfraglen, fragheaderlen, maxnonfragsize, xlength;
> + unsigned int maxfraglen, fragheaderlen, maxnonfragsize, initial_length;
> int csummode = CHECKSUM_NONE;
> struct rtable *rt = (struct rtable *)cork->dst;
> unsigned int wmem_alloc_delta = 0;
> @@ -1017,7 +1107,7 @@ static int __ip_append_data(struct sock *sk,
> (!exthdrlen || (rt->dst.dev->features & NETIF_F_HW_ESP_TX_CSUM)))
> csummode = CHECKSUM_PARTIAL;
>
> - xlength = length;
> + initial_length = length;
> if ((flags & MSG_ZEROCOPY) && length) {
> struct msghdr *msg = from;
>
> @@ -1053,7 +1143,7 @@ static int __ip_append_data(struct sock *sk,
> return -EPERM;
> if (!(rt->dst.dev->features & NETIF_F_SG))
> return -EOPNOTSUPP;
> - xlength = transhdrlen; /* We need an empty buffer to attach stuff to */
> + initial_length = transhdrlen; /* We need an empty buffer to attach stuff to */
> }
>
> cork->length += length;
> @@ -1083,47 +1173,13 @@ static int __ip_append_data(struct sock *sk,
> struct sk_buff *skb_prev;
>
> if (unlikely(flags & MSG_SPLICE_PAGES)) {
> - skb_prev = skb;
> - fraggap = skb_prev->len - maxfraglen;
> -
> - alloclen = fragheaderlen + hh_len + fraggap + 15;
> - skb = sock_wmalloc(sk, alloclen, 1, sk->sk_allocation);
> - if (unlikely(!skb)) {
> - err = -ENOBUFS;
> + err = __ip_splice_alloc(sk, &skb, fragheaderlen,
> + maxfraglen, hh_len);
> + if (err < 0)
> goto error;
> - }
> -
> - /*
> - * 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);
> continue;
> }
> - xlength = length;
> + initial_length = length;
>
> alloc_new_skb:
> skb_prev = skb;
> @@ -1136,7 +1192,7 @@ static int __ip_append_data(struct sock *sk,
> * If remaining data exceeds the mtu,
> * we know we need more fragment(s).
> */
> - datalen = xlength + fraggap;
> + datalen = initial_length + fraggap;
> if (datalen > mtu - fragheaderlen)
> datalen = maxfraglen - fragheaderlen;
> fraglen = datalen + fragheaderlen;
> @@ -1150,7 +1206,7 @@ static int __ip_append_data(struct sock *sk,
> * because we have no idea what fragment will be
> * the last.
> */
> - if (datalen == xlength + fraggap)
> + if (datalen == initial_length + fraggap)
> alloc_extra += rt->dst.trailer_len;
>
> if ((flags & MSG_MORE) &&
> @@ -1258,48 +1314,9 @@ static int __ip_append_data(struct sock *sk,
> goto error;
> }
> } else if (flags & MSG_SPLICE_PAGES) {
> - struct msghdr *msg = from;
> - struct page *page = NULL, **pages = &page;
> - size_t off;
> - bool put = false;
> -
> - copy = iov_iter_extract_pages(&msg->msg_iter, &pages,
> - copy, 1, 0, &off);
> - if (copy <= 0) {
> - err = copy ?: -EIO;
> - goto error;
> - }
> -
> - if (!sendpage_ok(page)) {
> - const void *p = kmap_local_page(page);
> - void *q;
> -
> - q = page_frag_memdup(NULL, p + off, copy,
> - sk->sk_allocation, ULONG_MAX);
> - kunmap_local(p);
> - if (!q) {
> - err = copy ?: -ENOMEM;
> - goto error;
> - }
> - page = virt_to_page(q);
> - off = offset_in_page(q);
> - put = true;
> - }
> -
> - err = skb_append_pagefrags(skb, page, off, copy);
> - if (put)
> - put_page(page);
> + err = __ip_splice_pages(sk, skb, from, &copy);
> if (err < 0)
> goto error;
> -
> - 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);
> } else if (!zc) {
> int i = skb_shinfo(skb)->nr_frags;
>