netdev - Re: [RFC] tcp md5 use of alloc

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Message-ID: <1414041833.2094.28.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 22 Oct 2014 22:23:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Ahern <dsahern@...il.com>
Cc:	Crestez Dan Leonard <cdleonard@...il.com>, netdev@...r.kernel.org
Subject: Re: [RFC] tcp md5 use of alloc_percpu

On Wed, 2014-10-22 at 22:40 -0600, David Ahern wrote:
> On 10/22/14, 12:55 PM, Crestez Dan Leonard wrote:
> > Hello,
> >
> > It seems that the TCP MD5 feature allocates a percpu struct tcp_md5sig_pool and uses part of that memory for a scratch buffer to do crypto on. Here is the relevant code:
> 
> This is a forward port of a local change to address the problem (local 
> kernel version is 3.4 so perhaps my quick bump to top of tree is off but 
> it shows the general idea). Been on my to-do list to figure out why this 
> is needed, but it seems related to your problem:
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 1bec4e76d88c..833a676bd4b0 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2941,7 +2941,7 @@ struct tcp_md5sig_pool *tcp_get_md5sig_pool(void)
>      local_bh_disable();
>      p = ACCESS_ONCE(tcp_md5sig_pool);
>      if (p)
> -       return raw_cpu_ptr(p);
> +       return __va(per_cpu_ptr_to_phys(raw_cpu_ptr(p)));
> 
>      local_bh_enable();
>      return NULL;

per_cpu_ptr_to_phys() can be pretty expensive and should not be called
in fast path.


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