netdev - Re: [PATCH v8] net: ethernet: add driver for Aurora VLSI NB8800 Ethernet controller

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Date:	Wed, 25 Nov 2015 16:16:14 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Mason <slash.tmp@...e.fr>, Netdev <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v8] net: ethernet: add driver for Aurora VLSI NB8800 Ethernet controller

Alexander Duyck <alexander.duyck@...il.com> writes:

> On Wed, Nov 25, 2015 at 5:04 AM, Måns Rullgård <mans@...sr.com> wrote:
>> Mason <slash.tmp@...e.fr> writes:
>>
>>> On 25/11/2015 13:45, Måns Rullgård wrote:
>>>
>>>> Mason wrote:
>>>>
>>>>> On 19/11/2015 14:02, Mans Rullgard wrote:
>>>>>
>>>>>> +  if (dma_mapping_error(&dev->dev, dma_addr)) {
>>>>>> +          skb_free_frag(data);
>>>>>> +          return -ENOMEM;
>>>>>> +  }
>>>>>
>>>>> I'm back-porting this driver to 4.1
>>>>>
>>>>> skb_free_frag() was introduced in 4.2 by 181edb2bfa22b IIUC.
>>>>>
>>>>> +static inline void skb_free_frag(void *addr)
>>>>> +{
>>>>> +       __free_page_frag(addr);
>>>>> +}
>>>>>
>>>>> Should I just copy the definition of __free_page_frag() ?
>>>>
>>>> Looks like it ought to work.  Try and find out.  Not that you'll ever
>>>> hit that error condition unless you fake it.
>>>
>>> Turns out __free_pages_ok() is static and I'd rather not touch
>>> mm/page_alloc.c in my back-port.
>>>
>>> Since you say the error condition is rare, I think I'll go with
>>> the code that 181edb2bfa22b replaced (put_page, IIUC).
>>>
>>> #include <linux/version.h>
>>> #if LINUX_VERSION_CODE < KERNEL_VERSION(4,2,0)
>>> #define skb_free_frag(data) put_page(virt_to_head_page(data))
>>> #else
>>> #error DELETE ME NOW (see commit 181edb2bfa22b)
>>> #endif
>>
>> You can simply put_page(page) instead since we already have the
>> virt_to_head_page() a few lines up.
>
> What you could do is use __free_pages instead of __free_pages_ok.
> Generally you will want to use __free_pages instead of put_page just
> to avoid a bunch of unnecessary tests and function pointer accesses.

Note that this is on a should-never-happen error path, so there's no
need to be counting cycles.

-- 
Måns Rullgård
mans@...sr.com
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