Re: [PATCH] kasan: call clear_page with a match-all tag instead of changing page tag

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Feb 15, 2023 at 10:47:26PM -0800, Peter Collingbourne wrote:
> Instead of changing the page's tag solely in order to obtain a pointer
> with a match-all tag and then changing it back again, just convert the
> pointer that we get from kmap_atomic() into one with a match-all tag
> before passing it to clear_page().
> 
> On a certain microarchitecture, this has been observed to cause a
> measurable improvement in microbenchmark performance, presumably as a
> result of being able to avoid the atomic operations on the page tag.

Yeah, this would likely break the write streaming mode on some ARM CPUs.

> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/I0249822cc29097ca7a04ad48e8eb14871f80e711
> ---
>  include/linux/highmem.h | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 44242268f53b..bbfa546dd602 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -245,12 +245,10 @@ static inline void clear_highpage(struct page *page)
>  
>  static inline void clear_highpage_kasan_tagged(struct page *page)
>  {
> -	u8 tag;
> +	void *kaddr = kmap_atomic(page);
>  
> -	tag = page_kasan_tag(page);
> -	page_kasan_tag_reset(page);
> -	clear_highpage(page);
> -	page_kasan_tag_set(page, tag);
> +	clear_page(kasan_reset_tag(kaddr));
> +	kunmap_atomic(kaddr);
>  }

Please don't add kmap_atomic() back. See commit d2c20e51e396
("mm/highmem: remove deprecated kmap_atomic"). I'd duplicate the
clear_highpage() logic in here and call clear_page() directly on the
address with the kasan tag reset.

-- 
Catalin

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