[RFC PATCH v1 04/57] mm/page_alloc: Make page_frag_cache boot-time page size compatible

[Vlastimil Babka]

On 10/14/24 12:58, Ryan Roberts wrote:
> "struct page_frag_cache" has some optimizations that depend on page
> size. Let's refactor it a bit so that those optimizations can be
> determined at run-time for the case where page size is a boot-time
> parameter. For compile-time page size, the compiler should dead code
> strip and the result is very similar to before.
> 
> One wrinkle is that we don't know if we need the size member until
> runtime. So remove the ifdeffery and always define offset as u32 (needed
> if PAGE_SIZE is >= 64K) and size as u16 (only used when PAGE_SIZE <=
> 32K). We move the members around a bit so that the overall size of the
> struct remains the same; 24 bytes for 64-bit and 16 bytes on 32 bit.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>

Looks ok, but ideally the PAGE_FRAG_CACHE_MAX_ORDER #define should also be
replaced by some variable that's populated just once. It can be static local
to page_alloc.c as nothing else seems to use it.

> 
> page_alloc
> ---
> 
> ***NOTE***
> Any confused maintainers may want to read the cover note here for context:
> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
> 
>  include/linux/mm_types.h | 13 ++++++-------
>  mm/page_alloc.c          | 31 ++++++++++++++++++-------------
>  2 files changed, 24 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 4854249792545..0844ed7cfaa53 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -544,16 +544,15 @@ static inline void *folio_get_private(struct folio *folio)
>  
>  struct page_frag_cache {
>  	void * va;
> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> -	__u16 offset;
> -	__u16 size;
> -#else
> -	__u32 offset;
> -#endif
>  	/* we maintain a pagecount bias, so that we dont dirty cache line
>  	 * containing page->_refcount every time we allocate a fragment.
>  	 */
> -	unsigned int		pagecnt_bias;
> +	unsigned int pagecnt_bias;
> +	__u32 offset;
> +	/* size only used when PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE, in which
> +	 * case PAGE_FRAG_CACHE_MAX_SIZE is 32K and 16 bits is sufficient.
> +	 */
> +	__u16 size;
>  	bool pfmemalloc;
>  };
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 91ace8ca97e21..8678103b1b396 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4822,13 +4822,18 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
>  	struct page *page = NULL;
>  	gfp_t gfp = gfp_mask;
>  
> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> -	gfp_mask = (gfp_mask & ~__GFP_DIRECT_RECLAIM) |  __GFP_COMP |
> -		   __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC;
> -	page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
> -				PAGE_FRAG_CACHE_MAX_ORDER);
> -	nc->size = page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE;
> -#endif
> +	if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) {
> +		gfp_mask = (gfp_mask & ~__GFP_DIRECT_RECLAIM) |  __GFP_COMP |
> +			   __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC;
> +		page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
> +					PAGE_FRAG_CACHE_MAX_ORDER);
> +		/*
> +		 * Cast to silence warning due to 16-bit nc->size. Not real
> +		 * because PAGE_SIZE only less than PAGE_FRAG_CACHE_MAX_SIZE
> +		 * when PAGE_FRAG_CACHE_MAX_SIZE is 32K.
> +		 */
> +		nc->size = (__u16)(page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE);
> +	}
>  	if (unlikely(!page))
>  		page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
>  
> @@ -4870,10 +4875,10 @@ 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
> +		if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> +			size = nc->size;
> +
>  		/* Even if we own the page, we do not use atomic_set().
>  		 * This would break get_page_unless_zero() users.
>  		 */
> @@ -4897,10 +4902,10 @@ 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
> +		if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> +			size = nc->size;
> +
>  		/* OK, page count is 0, we can safely set it */
>  		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
>