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

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Date:   Tue, 04 Apr 2023 12:58:25 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     David Howells <dhowells@...hat.com>,
        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

David Howells wrote:
> 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.

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@...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;
>  
>