Subject: Re: [RFC] tcp md5 use of alloc_percpu

From: Jonathan Toppins <[email-masked]>
Date: Thu, 23 Oct 2014 02:58:53 -0400

On 10/23/14, 1:38 AM, Eric Dumazet wrote:
> On Wed, 2014-10-22 at 22:23 -0700, Eric Dumazet wrote:
>> 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.
>>
> 
> My updated patch would be :
> 
>  net/ipv4/tcp.c |   66 +++++++++++++++++++----------------------------
>  1 file changed, 28 insertions(+), 38 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 1bec4e76d88c..af4dc16b61f6 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2868,61 +2868,51 @@ EXPORT_SYMBOL(compat_tcp_getsockopt);
>  #endif
>  
>  #ifdef CONFIG_TCP_MD5SIG
> -static struct tcp_md5sig_pool __percpu *tcp_md5sig_pool __read_mostly;
> +static DEFINE_PER_CPU(struct tcp_md5sig_pool, *tcp_md5sig_pool);
>  static DEFINE_MUTEX(tcp_md5sig_mutex);
> -
> -static void __tcp_free_md5sig_pool(struct tcp_md5sig_pool __percpu *pool)
> -{
> -	int cpu;
> -
> -	for_each_possible_cpu(cpu) {
> -		struct tcp_md5sig_pool *p = per_cpu_ptr(pool, cpu);
> -
> -		if (p->md5_desc.tfm)
> -			crypto_free_hash(p->md5_desc.tfm);
> -	}
> -	free_percpu(pool);
> -}
> +static bool tcp_md5sig_pool_populated = false;
>  
>  static void __tcp_alloc_md5sig_pool(void)
>  {
>  	int cpu;
> -	struct tcp_md5sig_pool __percpu *pool;
> -
> -	pool = alloc_percpu(struct tcp_md5sig_pool);
> -	if (!pool)
> -		return;
>  
>  	for_each_possible_cpu(cpu) {
> +		struct tcp_md5sig_pool *pool;
>  		struct crypto_hash *hash;
>  
> -		hash = crypto_alloc_hash("md5", 0, CRYPTO_ALG_ASYNC);
> -		if (IS_ERR_OR_NULL(hash))
> -			goto out_free;
> -
> -		per_cpu_ptr(pool, cpu)->md5_desc.tfm = hash;
> +		pool = per_cpu(tcp_md5sig_pool, cpu);
> +		if (!pool) {
> +			pool = kzalloc_node(sizeof(*pool), GFP_KERNEL,
GFP_DMA | GFP_KERNEL
This memory will possibly be used in a DMA correct? (thinking crypto
hardware offload)
> +					    cpu_to_node(cpu));
> +			if (!pool)
> +				return;
> +			per_cpu(tcp_md5sig_pool, cpu) = pool;
> +		}
> +		if (!pool->md5_desc.tfm) {
> +			hash = crypto_alloc_hash("md5", 0, CRYPTO_ALG_ASYNC);
> +			if (IS_ERR_OR_NULL(hash))
> +				return;
> +			pool->md5_desc.tfm = hash;
> +		}
>  	}
> -	/* before setting tcp_md5sig_pool, we must commit all writes
> -	 * to memory. See ACCESS_ONCE() in tcp_get_md5sig_pool()
> +	/* before setting tcp_md5sig_pool_populated, we must commit all writes
> +	 * to memory. See smp_rmb() in tcp_get_md5sig_pool()
>  	 */
>  	smp_wmb();
> -	tcp_md5sig_pool = pool;
> -	return;
> -out_free:
> -	__tcp_free_md5sig_pool(pool);
> +	tcp_md5sig_pool_populated = true;
>  }
>  
>  bool tcp_alloc_md5sig_pool(void)
>  {
> -	if (unlikely(!tcp_md5sig_pool)) {
> +	if (unlikely(!tcp_md5sig_pool_populated)) {
>  		mutex_lock(&tcp_md5sig_mutex);
>  
> -		if (!tcp_md5sig_pool)
> +		if (!tcp_md5sig_pool_populated)
>  			__tcp_alloc_md5sig_pool();
>  
>  		mutex_unlock(&tcp_md5sig_mutex);
>  	}
> -	return tcp_md5sig_pool != NULL;
> +	return tcp_md5sig_pool_populated;
>  }
>  EXPORT_SYMBOL(tcp_alloc_md5sig_pool);
>  
> @@ -2936,13 +2926,13 @@ EXPORT_SYMBOL(tcp_alloc_md5sig_pool);
>   */
>  struct tcp_md5sig_pool *tcp_get_md5sig_pool(void)
>  {
> -	struct tcp_md5sig_pool __percpu *p;
> -
>  	local_bh_disable();
> -	p = ACCESS_ONCE(tcp_md5sig_pool);
> -	if (p)
> -		return raw_cpu_ptr(p);
>  
> +	if (tcp_md5sig_pool_populated) {
> +		/* coupled with smp_wmb() in __tcp_alloc_md5sig_pool */
> +		smp_rmb();
> +		return this_cpu_read(tcp_md5sig_pool);
> +	}
>  	local_bh_enable();
>  	return NULL;
>  }
> 
> 
> 
> --
> To unsubscribe from this list: send the line "unsubscribe netdev" in
> the body of a message to [email-masked]
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>