Subject: Re: [PATCH net-next v5 01/27] ipv4: avoid partial copy for zc

From: Jakub Kicinski <[email-masked]>
Date: Mon, 18 Jul 2022 18:54:13 -0700

On Tue, 12 Jul 2022 21:52:25 +0100 Pavel Begunkov wrote:
> Even when zerocopy transmission is requested and possible,
> __ip_append_data() will still copy a small chunk of data just because it
> allocated some extra linear space (e.g. 148 bytes). It wastes CPU cycles
> on copy and iter manipulations and also misalignes potentially aligned
> data. Avoid such coies. And as a bonus we can allocate smaller skb.

s/coies/copies/ can fix when applying

> 
> Signed-off-by: Pavel Begunkov <[email-masked]>
> ---
>  net/ipv4/ip_output.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 00b4bf26fd93..581d1e233260 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -969,7 +969,6 @@ static int __ip_append_data(struct sock *sk,
>  	struct inet_sock *inet = inet_sk(sk);
>  	struct ubuf_info *uarg = NULL;
>  	struct sk_buff *skb;
> -
>  	struct ip_options *opt = cork->opt;
>  	int hh_len;
>  	int exthdrlen;
> @@ -977,6 +976,7 @@ static int __ip_append_data(struct sock *sk,
>  	int copy;
>  	int err;
>  	int offset = 0;
> +	bool zc = false;
>  	unsigned int maxfraglen, fragheaderlen, maxnonfragsize;
>  	int csummode = CHECKSUM_NONE;
>  	struct rtable *rt = (struct rtable *)cork->dst;
> @@ -1025,6 +1025,7 @@ static int __ip_append_data(struct sock *sk,
>  		if (rt->dst.dev->features & NETIF_F_SG &&
>  		    csummode == CHECKSUM_PARTIAL) {
>  			paged = true;
> +			zc = true;
>  		} else {
>  			uarg->zerocopy = 0;
>  			skb_zcopy_set(skb, uarg, &extra_uref);
> @@ -1091,9 +1092,12 @@ static int __ip_append_data(struct sock *sk,
>  				 (fraglen + alloc_extra < SKB_MAX_ALLOC ||
>  				  !(rt->dst.dev->features & NETIF_F_SG)))
>  				alloclen = fraglen;
> -			else {
> +			else if (!zc) {
>  				alloclen = min_t(int, fraglen, MAX_HEADER);

Willem, I think this came in with your GSO work, is there a reason we
use MAX_HEADER here? I thought MAX_HEADER is for headers (i.e. more or
less to be reserved) not for the min amount of data to be included.

I wanna make sure we're not missing something about GSO here.

Otherwise I don't think we need the extra branch but that can 
be a follow up.

>  				pagedlen = fraglen - alloclen;
> +			} else {
> +				alloclen = fragheaderlen + transhdrlen;
> +				pagedlen = datalen - transhdrlen;
>  			}
>  
>  			alloclen += alloc_extra;