netdev - Re: [PATCH V3 for 3.19] rtlwifi: Fix error when accessing unmapped memory in skb

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Message-ID: <54A4665A.8090906@lwfinger.net>
Date:	Wed, 31 Dec 2014 15:10:50 -0600
From:	Larry Finger <Larry.Finger@...inger.net>
To:	Eric Biggers <ebiggers3@...il.com>
CC:	kvalo@...eaurora.org, linux-wireless@...r.kernel.org,
	netdev@...r.kernel.org, Stable <stable@...r.kernel.org>
Subject: Re: [PATCH V3 for 3.19]  rtlwifi: Fix error when accessing unmapped
 memory in skb

On 12/30/2014 11:07 PM, Eric Biggers wrote:
> On Tue, Dec 30, 2014 at 09:33:07PM -0600, Larry Finger wrote:
>> v3 - Unmap skb before trying to allocate a new one so as to not leak mapping.
>
> Looks good to me, although I'm not sure about the handling of DMA mapping errors
> (perhaps that's something that drivers typically don't even try to handle?).
> Anyway, the skb allocation issue appears to be resolved now.  I am running your
> patch with an extra hack to inject some occasional skb allocation failures, and
> I haven't noticed any problems except dropped packets.

The last time I saw any DMA mapping errors were for some early BCM43xx cards 
that only had 20 bits of DMA addressing space. These Realtek devices have a full 
32 bits of addressing, thus any physical address in the first 4GB of RAM will be 
OK. I suppose that it might be possible to get a physical address outside this 
range for machines with a lot of RAM, but they are unlikely to have wifi interfaces.

Thanks for the testing. The Realtek engineer told me that they are looking at 
this section, and may do a rewrite. I'm waiting to see what happens there before 
considering alternatives. If the number of packets dropped due to skb allocation 
failures is small, then the current code is likely OK.

Larry

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