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

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Date:	Wed, 25 Nov 2015 08:12:28 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Måns Rullgård <mans@...sr.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

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.

The result would look something like:
static inline void skb_free_frag(void *addr)
{
       struct page *page = virt_to_head_page(addr);

        __free_pages(page, compound_order(page));
}

Hope that helps.

- Alex
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