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

[Alexander H Duyck <alexander.duyck@gmail.com>]

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".

>  		page = virt_to_page(nc->va);
>  
>  		if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
> @@ -104,17 +106,13 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>  			goto refill;
>  		}
>  
> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> -		/* if size can vary use size else just use PAGE_SIZE */
> -		size = nc->size;
> -#endif
>  		/* OK, page count is 0, we can safely set it */
>  		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
>  
>  		/* reset page count bias and offset to start of new frag */
>  		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
> -		offset = size - fragsz;
> -		if (unlikely(offset < 0)) {
> +		offset = 0;
> +		if (unlikely(fragsz > size)) {
>  			/*
>  			 * The caller is trying to allocate a fragment
>  			 * with fragsz > PAGE_SIZE but the cache isn't big
> @@ -129,8 +127,7 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>  	}
>  
>  	nc->pagecnt_bias--;
> -	offset &= align_mask;
> -	nc->offset = offset;
> +	nc->offset = offset + fragsz;
>  
>  	return nc->va + offset;
>  }