Re: [PATCH 2/2] tcp: cleanup tcp_try_coalesce

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2012-05-02 at 20:39 -0700, Alexander Duyck wrote:
> This change is mostly meant to help improve the readability of
> tcp_try_coalesce.  I had a few issues since there were several points when
> the code would test for a conditional, fail, then succeed on another
> conditional take some action, and then follow a goto back into the previous
> conditional.  I just tore all of that apart and made the whole thing one
> linear flow with a single goto.
> 
> Also there were multiple ways of computing the delta, the one for head_frag
> made the least amount of sense to me since we were only dropping the
> sk_buff so I have updated the logic for the stolen head case so that delta
> is only truesize - sizeof(skb_buff), and for the case where we are dropping
> the head as well it is truesize - SKB_TRUESIZE(skb_end_pointer - head).
> This way we can also account for the head_frag with headlen == 0.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
> ---
> 

Sorry I prefer you dont touch this code like this.

The truesize bits must stay as is, since it'll track drivers that lies
about truesize.

>  net/ipv4/tcp_input.c |   80 +++++++++++++++++++++++++++-----------------------
>  1 files changed, 43 insertions(+), 37 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index c6f78e2..23bc3ff 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4548,62 +4548,68 @@ static bool tcp_try_coalesce(struct sock *sk,
>  	int i, delta, len = from->len;
>  
>  	*fragstolen = false;
> +
>  	if (tcp_hdr(from)->fin || skb_cloned(to))
>  		return false;
> +
>  	if (len <= skb_tailroom(to)) {
>  		BUG_ON(skb_copy_bits(from, 0, skb_put(to, len), len));
> -merge:
> -		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
> -		TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
> -		TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
> -		return true;
> +		goto merge;
>  	}
>  
>  	if (skb_has_frag_list(to) || skb_has_frag_list(from))
>  		return false;
>  
> -	if (skb_headlen(from) == 0 &&
> -	    (skb_shinfo(to)->nr_frags +
> -	     skb_shinfo(from)->nr_frags <= MAX_SKB_FRAGS)) {
> -		WARN_ON_ONCE(from->head_frag);
> -		delta = from->truesize - ksize(from->head) -
> -			SKB_DATA_ALIGN(sizeof(struct sk_buff));


This delta was done on purpose. We want the ksize()

> -
> -		WARN_ON_ONCE(delta < len);
> -copyfrags:
> -		memcpy(skb_shinfo(to)->frags + skb_shinfo(to)->nr_frags,
> -		       skb_shinfo(from)->frags,
> -		       skb_shinfo(from)->nr_frags * sizeof(skb_frag_t));
> -		skb_shinfo(to)->nr_frags += skb_shinfo(from)->nr_frags;
> -
> -		if (skb_cloned(from))
> -			for (i = 0; i < skb_shinfo(from)->nr_frags; i++)
> -				skb_frag_ref(from, i);
> -		else
> -			skb_shinfo(from)->nr_frags = 0;
> -
> -		to->truesize += delta;
> -		atomic_add(delta, &sk->sk_rmem_alloc);
> -		sk_mem_charge(sk, delta);
> -		to->len += len;
> -		to->data_len += len;
> -		goto merge;
> -	}
> -	if (from->head_frag && !skb_cloned(from)) {
> +	if (skb_headlen(from) != 0) {
>  		struct page *page;
>  		unsigned int offset;
>  
> -		if (skb_shinfo(to)->nr_frags + skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
> +		if (skb_shinfo(to)->nr_frags +
> +		    skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
> +			return false;
> +
> +		if (!from->head_frag || skb_cloned(from))
>  			return false;
> +
> +		delta = from->truesize - sizeof(struct sk_buff);



> +
>  		page = virt_to_head_page(from->head);
>  		offset = from->data - (unsigned char *)page_address(page);
> +
>  		skb_fill_page_desc(to, skb_shinfo(to)->nr_frags,
>  				   page, offset, skb_headlen(from));
>  		*fragstolen = true;
> -		delta = len; /* we dont know real truesize... */
> -		goto copyfrags;
> +	} else {
> +		if (skb_shinfo(to)->nr_frags +
> +		    skb_shinfo(from)->nr_frags > MAX_SKB_FRAGS)
> +			return false;
> +
> +		delta = from->truesize -
> +			SKB_TRUESIZE(skb_end_pointer(from) - from->head);

No... SKB_TRUESIZE() doesnt account of power-of-two roundings in
kmalloc()

>  	}
> -	return false;
> +
> +	memcpy(skb_shinfo(to)->frags + skb_shinfo(to)->nr_frags,
> +	       skb_shinfo(from)->frags,
> +	       skb_shinfo(from)->nr_frags * sizeof(skb_frag_t));
> +	skb_shinfo(to)->nr_frags += skb_shinfo(from)->nr_frags;
> +
> +	if (!skb_cloned(from))
> +		skb_shinfo(from)->nr_frags = 0;
> +

You break the code here... we had an else.

> +	for (i = 0; i < skb_shinfo(from)->nr_frags; i++)
> +		skb_frag_ref(from, i);
> +
> +	to->truesize += delta;
> +	atomic_add(delta, &sk->sk_rmem_alloc);
> +	sk_mem_charge(sk, delta);
> +	to->len += len;
> +	to->data_len += len;
> +
> +merge:
> +	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
> +	TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
> +	TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
> +	return true;
>  }
>  
>  static void kfree_skb_partial(struct sk_buff *skb, bool head_stolen)
> 

Really this patch is too hard to review.

Thanks