Re: [net PATCH 1/2] mm: Use fixed constant in page_frag_alloc instead of size + 1

[Vlastimil Babka <vbabka@suse.cz>]

On 2/15/19 23:44, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> This patch replaces the size + 1 value introduced with the recent fix for 1
> byte allocs with a constant value.
> 
> The idea here is to reduce code overhead as the previous logic would have
> to read size into a register, then increment it, and write it back to
> whatever field was being used. By using a constant we can avoid those
> memory reads and arithmetic operations in favor of just encoding the
> maximum value into the operation itself.
> 
> Fixes: 2c2ade81741c ("mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs")
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  mm/page_alloc.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ebb35e4d0d90..37ed14ad0b59 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4857,11 +4857,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
>  		/* Even if we own the page, we do not use atomic_set().
>  		 * This would break get_page_unless_zero() users.
>  		 */
> -		page_ref_add(page, size);
> +		page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE);

But this value can be theoretically too low when PAGE_SIZE >
PAGE_FRAG_CACHE_MAX_SIZE? Such as on architectures with 64kB page size,
while PAGE_FRAG_CACHE_MAX_SIZE is 32kB?

Maybe impossible to exploit in practice thanks to the minimum alignment, but
still IMHO we should be using the larger of PAGE_FRAG_CACHE_MAX_SIZE and
PAGE_SIZE, which should still be a build-time constant, so not defeat the
optimization.

>  
>  		/* reset page count bias and offset to start of new frag */
>  		nc->pfmemalloc = page_is_pfmemalloc(page);
> -		nc->pagecnt_bias = size + 1;
> +		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>  		nc->offset = size;
>  	}
>  
> @@ -4877,10 +4877,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
>  		size = nc->size;
>  #endif
>  		/* OK, page count is 0, we can safely set it */
> -		set_page_count(page, size + 1);
> +		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
>  
>  		/* reset page count bias and offset to start of new frag */
> -		nc->pagecnt_bias = size + 1;
> +		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>  		offset = size - fragsz;
>  	}
>  
>