Re: [PATCH net-next v12 1/5] page_pool: unify frag_count handling in page_pool_is_last_frag()

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2023/10/23 19:43, Ilias Apalodimas wrote:
> Hi Yunsheng, 
> 
> [...]
> 
>> +	 * 1. 'n == 1': no need to actually overwrite it.
>> +	 * 2. 'n != 1': overwrite it with one, which is the rare case
>> +	 *              for pp_frag_count draining.
>>  	 *
>> -	 * The main advantage to doing this is that an atomic_read is
>> -	 * generally a much cheaper operation than an atomic update,
>> -	 * especially when dealing with a page that may be partitioned
>> -	 * into only 2 or 3 pieces.
>> +	 * The main advantage to doing this is that not only we avoid a atomic
>> +	 * update, as an atomic_read is generally a much cheaper operation than
>> +	 * an atomic update, especially when dealing with a page that may be
>> +	 * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count
>> +	 * handling by ensuring all pages have partitioned into only 1 piece
>> +	 * initially, and only overwrite it when the page is partitioned into
>> +	 * more than one piece.
>>  	 */
>> -	if (atomic_long_read(&page->pp_frag_count) == nr)
>> +	if (atomic_long_read(&page->pp_frag_count) == nr) {
>> +		/* As we have ensured nr is always one for constant case using
>> +		 * the BUILD_BUG_ON(), only need to handle the non-constant case
>> +		 * here for pp_frag_count draining, which is a rare case.
>> +		 */
>> +		BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
>> +		if (!__builtin_constant_p(nr))
>> +			atomic_long_set(&page->pp_frag_count, 1);
> 
> Aren't we changing the behaviour of the current code here? IIRC is
> atomic_long_read(&page->pp_frag_count) == nr we never updated the atomic
> pp_frag_count and the reasoning was that the next caller can set it
> properly. 

If the next caller is calling the page_pool_alloc_frag(), then yes,
because page_pool_fragment_page() will be used to reset the
page->pp_frag_count, so it does not really matter what is the value
of page->pp_frag_count when we are recycling a page.

If the next caller is calling page_pool_alloc_pages() directly without
fragmenting a page, the above code is used to ensure that pp_frag_count
is always one when page_pool_alloc_pages() fetches a page from pool->alloc
or pool->ring, because page_pool_fragment_page() is not used to reset the
page->pp_frag_count for page_pool_alloc_pages() and we have removed the
per page_pool PP_FLAG_PAGE_FRAG in page_pool_is_last_frag().

As we don't know if the caller is page_pool_alloc_frag() or
page_pool_alloc_pages(), so the above code ensure the page in pool->alloc
or pool->ring always have the pp_frag_count being one.


> 
>> +
>>  		return 0;
>> +	}
>>  
>>  	ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>>  	WARN_ON(ret < 0);
>> +
>> +	/* We are the last user here too, reset pp_frag_count back to 1 to
>> +	 * ensure all pages have been partitioned into 1 piece initially,
>> +	 * this should be the rare case when the last two fragment users call
>> +	 * page_pool_defrag_page() currently.
>> +	 */
>> +	if (unlikely(!ret))
>> +		atomic_long_set(&page->pp_frag_count, 1);
>> +
>>  	return ret;
>>  }
>>  
>  
>  [....]
> 
>  Thanks
>  /Ilias
> 
> .
>