netdev - Re: 3.14-mw regression: rtl8169 WARNING: DMA-API: exceeded 7 overlapping mappings of pfn 55ebe

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Date:	Wed, 12 Feb 2014 22:52:42 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Sander Eikelenboom <linux@...elenboom.it>,
	Dan Williams <dan.j.williams@...el.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Francois Romieu <romieu@...zoreil.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 3.14-mw regression: rtl8169 WARNING: DMA-API: exceeded 7
 overlapping mappings of pfn 55ebe

On Tue, 2014-02-11 at 13:28 -0800, Eric Dumazet wrote:
[...]
> Incoming frames might be taken out of order-3 pages.
> 
> With regular Ethernet frames, this is 21 frames per order-3 pages.
> 
> ACTIVE_PFN_MAX_OVERLAP seems too small.
> 
> Alternative would be to user order-0 only pages if CONFIG_DMA_API_DEBUG
> is set. Not sure if it works if PAGE_SIZE=66536 ....

Indeed, you can get a lot of packet buffers into a 64K page...

> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index f589c9af8cbf..1b9995adfd29 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1924,7 +1924,11 @@ static inline void __skb_queue_purge(struct sk_buff_head *list)
>  		kfree_skb(skb);
>  }
>  
> +#if defined(CONFIG_DMA_API_DEBUG)
> +#define NETDEV_FRAG_PAGE_MAX_ORDER 0
> +#else
>  #define NETDEV_FRAG_PAGE_MAX_ORDER get_order(32768)
> +#endif
>  #define NETDEV_FRAG_PAGE_MAX_SIZE  (PAGE_SIZE << NETDEV_FRAG_PAGE_MAX_ORDER)
>  #define NETDEV_PAGECNT_MAX_BIAS	   NETDEV_FRAG_PAGE_MAX_SIZE
>  

That may be useful for debugging this particular problem, but please
don't make debugging options change behaviour like this.

Ben.

-- 
Ben Hutchings
If more than one person is responsible for a bug, no one is at fault.

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