netdev - Re: [PATCH net-next,v4] net/tls: Calculate nsg for zerocopy path without skb_cow

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Message-ID: <20180809224344.GA48089@doronrk-mbp.dhcp.thefacebook.com>
Date:   Thu, 9 Aug 2018 15:43:44 -0700
From:   Doron Roberts-Kedes <doronrk@...com>
To:     David Miller <davem@...emloft.net>
CC:     <davejwatson@...com>, <vakul.garg@....com>, <borisp@...lanox.com>,
        <aviadye@...lanox.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next,v4] net/tls: Calculate nsg for zerocopy path
 without skb_cow_data.

On Wed, Aug 08, 2018 at 12:14:30PM -0700, David Miller wrote:
> From: Doron Roberts-Kedes <doronrk@...com>
> Date: Tue, 7 Aug 2018 11:09:39 -0700
> 
> > +static int __skb_nsg(struct sk_buff *skb, int offset, int len,
> > +		     unsigned int recursion_level)
> > +{
> > +	int start = skb_headlen(skb);
> > +	int i, copy = start - offset;
> > +	struct sk_buff *frag_iter;
> > +	int elt = 0;
> > +
> > +	if (unlikely(recursion_level >= 24))
> > +		return -EMSGSIZE;
> 
> This recursion is kinda crazy.
> 
> Even skb_cow_data() doesn't recurse like this (of course because it copies
> into linear buffers).
> 
> There has to be a way to simplify this.  Fragment lists are such a rarely
> used SKB geometry, and few if any devices support it for transmission
> (so the fraglist will get undone at transmit time anyways).
> 

Interesting. Just wanted to clarify whether the issue is the use of
recursion or the fact that the function is handling the frag_list at
all. This is the rx path, so my understanding was that we need to handle
the frag_list. Please let me know if I'm misunderstanding your point
about the rare use of fragment lists.

If the issue is the recursion, I can rewrite the function to not use
recursion, but skb_to_sgvec uses a similar pattern and is invoked
immediately afterwards.

Taking a step back, is there an existing solution for what this function
is trying to do? I was surprised to find that there did not seem to
exist a function for determining the number of scatterlist elements
required to map an skb without COW.