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Subject: Re: [PATCH rfc v6 2/4] page_pool: add interface to manipulate frag
 count in page pool
To:     Alexander Duyck <alexander.duyck@gmail.com>
CC:     David Miller <davem@davemloft.net>,
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From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <fffae41f-b0a3-3c43-491f-096d31ba94ca@huawei.com>
Date:   Thu, 22 Jul 2021 16:07:48 +0800
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On 2021/7/21 22:06, Alexander Duyck wrote:
> On Wed, Jul 21, 2021 at 1:15 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>
>> On 2021/7/20 23:43, Alexander Duyck wrote:
>>> On Mon, Jul 19, 2021 at 8:36 PM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>>>
>>>> For 32 bit systems with 64 bit dma, dma_addr[1] is used to
>>>> store the upper 32 bit dma addr, those system should be rare
>>>> those days.
>>>>
>>>> For normal system, the dma_addr[1] in 'struct page' is not
>>>> used, so we can reuse dma_addr[1] for storing frag count,
>>>> which means how many frags this page might be splited to.
>>>>
>>>> In order to simplify the page frag support in the page pool,
>>>> the PAGE_POOL_DMA_USE_PP_FRAG_COUNT macro is added to indicate
>>>> the 32 bit systems with 64 bit dma, and the page frag support
>>>> in page pool is disabled for such system.
>>>>
>>>> The newly added page_pool_set_frag_count() is called to reserve
>>>> the maximum frag count before any page frag is passed to the
>>>> user. The page_pool_atomic_sub_frag_count_return() is called
>>>> when user is done with the page frag.
>>>>
>>>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>>>> ---
>>>>  include/linux/mm_types.h | 18 +++++++++++++-----
>>>>  include/net/page_pool.h  | 41 ++++++++++++++++++++++++++++++++++-------
>>>>  net/core/page_pool.c     |  4 ++++
>>>>  3 files changed, 51 insertions(+), 12 deletions(-)
>>>>
>>>
>>> <snip>
>>>
>>>> +static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
>>>> +                                                         long nr)
>>>> +{
>>>> +       long frag_count = atomic_long_read(&page->pp_frag_count);
>>>> +       long ret;
>>>> +
>>>> +       if (frag_count == nr)
>>>> +               return 0;
>>>> +
>>>> +       ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>>>> +       WARN_ON(ret < 0);
>>>> +       return ret;
>>>>  }
>>>>
>>>
>>> So this should just be an atomic_long_sub_return call. You should get
>>> rid of the atomic_long_read portion of this as it can cover up
>>> reference count errors.
>>
>> atomic_long_sub_return() is used to avoid one possible cache bouncing and
>> barrrier caused by the last user.
> 
> I assume you mean "atomic_long_read()" here.

Yes, sorry for the confusion.

> 
>> You are right that that may cover up the reference count errors. How about
>> something like below:
>>
>> static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
>>                                                           long nr)
>> {
>> #ifdef CONFIG_DEBUG_PAGE_REF
>>         long ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>>
>>         WARN_ON(ret < 0);
>>
>>         return ret;
>> #else
>>         if (atomic_long_read(&page->pp_frag_count) == nr)
>>                 return 0;
>>
>>         return atomic_long_sub_return(nr, &page->pp_frag_count);
>> #end
>> }
>>
>> Or any better suggestion?
> 
> So the one thing I might change would be to make it so that you only
> do the atomic_long_read if nr is a constant via __builtin_constant_p.
> That way you would be performing the comparison in
> __page_pool_put_page and in the cases of freeing or draining the
> page_frags you would be using the atomic_long_sub_return which should
> be paths where you would not expect it to match or that are slowpath
> anyway.
> 
> Also I would keep the WARN_ON in both paths just to be on the safe side.

If I understand it correctly, we should change it as below, right?

static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
							  long nr)
{
	long ret;

	/* As suggested by Alexander, atomic_long_read() may cover up the
	 * reference count errors, so avoid calling atomic_long_read() in
	 * the cases of freeing or draining the page_frags, where we would
	 * not expect it to match or that are slowpath anyway.
	 */
	if (__builtin_constant_p(nr) &&
	    atomic_long_read(&page->pp_frag_count) == nr)
		return 0;

	ret = atomic_long_sub_return(nr, &page->pp_frag_count);
	WARN_ON(ret < 0);
	return ret;
}


> .
>