Re: [PATCH net-next v1 02/12] mm: page_frag: use initial zero offset for page_frag_alloc_align()

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/4/8 1:52, Alexander H Duyck wrote:
> On Sun, 2024-04-07 at 21:08 +0800, Yunsheng Lin wrote:
>> We are above to use page_frag_alloc_*() API to not just
>> allocate memory for skb->data, but also use them to do
>> the memory allocation for skb frag too. Currently the
>> implementation of page_frag in mm subsystem is running
>> the offset as a countdown rather than count-up value,
>> there may have several advantages to that as mentioned
>> in [1], but it may have some disadvantages, for example,
>> it may disable skb frag coaleasing and more correct cache
>> prefetching
>>
>> We have a trade-off to make in order to have a unified
>> implementation and API for page_frag, so use a initial zero
>> offset in this patch, and the following patch will try to
>> make some optimization to aovid the disadvantages as much
>> as possible.
>>
>> 1. https://lore.kernel.org/all/f4abe71b3439b39d17a6fb2d410180f367cadf5c.camel@gmail.com/
>>
>> CC: Alexander Duyck <alexander.duyck@gmail.com>
>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>> ---
>>  mm/page_frag_cache.c | 31 ++++++++++++++-----------------
>>  1 file changed, 14 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
>> index a0f90ba25200..3e3e88d9af90 100644
>> --- a/mm/page_frag_cache.c
>> +++ b/mm/page_frag_cache.c
>> @@ -67,9 +67,8 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>>  			      unsigned int fragsz, gfp_t gfp_mask,
>>  			      unsigned int align_mask)
>>  {
>> -	unsigned int size = PAGE_SIZE;
>> +	unsigned int size, offset;
>>  	struct page *page;
>> -	int offset;
>>  
>>  	if (unlikely(!nc->va)) {
>>  refill:
>> @@ -77,10 +76,6 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>>  		if (!page)
>>  			return NULL;
>>  
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> -		/* if size can vary use size else just use PAGE_SIZE */
>> -		size = nc->size;
>> -#endif
>>  		/* Even if we own the page, we do not use atomic_set().
>>  		 * This would break get_page_unless_zero() users.
>>  		 */
>> @@ -89,11 +84,18 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>>  		/* reset page count bias and offset to start of new frag */
>>  		nc->pfmemalloc = page_is_pfmemalloc(page);
>>  		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>> -		nc->offset = size;
>> +		nc->offset = 0;
>>  	}
>>  
>> -	offset = nc->offset - fragsz;
>> -	if (unlikely(offset < 0)) {
>> +#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> +	/* if size can vary use size else just use PAGE_SIZE */
>> +	size = nc->size;
>> +#else
>> +	size = PAGE_SIZE;
>> +#endif
>> +
>> +	offset = ALIGN(nc->offset, -align_mask);
>> +	if (unlikely(offset + fragsz > size)) {
> 
> Rather than using "ALIGN" with a negative value it would probably make
> more sense to use __ALIGN_KERNEL_MASK with ~align_mask. I am not sure
> how well the compiler sorts out the use of negatives to flip values
> that are then converted to masks with the "(a) - 1".

The next patch will remove the '-' in '-align_mask' as the 'ALIGN' operation
is done in the inline helper. I am not sure that matter much to use
__ALIGN_KERNEL_MASK with ~align_mask?

>