Re: [PATCH V2] mm: alloc_tag: change the KASAN_TAG_WIDTH for HW_TAGS

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Apr 28, 2025 at 4:00 AM Guilherme Giacomo Simoes
<trintaeoitogc@gmail.com> wrote:
>
> the KASAN_TAG_WIDTH is 8 bits for both (HW_TAGS and SW_TAGS), but for
> HW_TAGS the KASAN_TAG_WIDTH can be 4 bits bits because due to the design
> of the MTE the memory words for storing metadata only need 4 bits.
> Change the preprocessor define KASAN_TAG_WIDTH for check if SW_TAGS is
> define, so KASAN_TAG_WIDTH should be 8 bits, but if HW_TAGS is define,
> so KASAN_TAG_WIDTH should be 4 bits to save a few flags bits.

The title lists "alloc_tag" but the change does not directly relate to
allocation tags. I think it should be page-flags-layout.h instead.

>
> Suggested-by: Andrey Konovalov <andreyknvl@gmail.com>
> Signed-off-by: Guilherme Giacomo Simoes <trintaeoitogc@gmail.com>
> ---
>
> V2 changes:
> - Improve commit message
>
> ---
>  include/linux/page-flags-layout.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
> index 4f5c9e979bb9..760006b1c480 100644
> --- a/include/linux/page-flags-layout.h
> +++ b/include/linux/page-flags-layout.h
> @@ -72,8 +72,10 @@
>  #define NODE_NOT_IN_PAGE_FLAGS 1
>  #endif
>
> -#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
> +#if defined(CONFIG_KASAN_SW_TAGS)
>  #define KASAN_TAG_WIDTH 8
> +#elif defined(CONFIG_KASAN_HW_TAGS)
> +#define KASAN_TAG_WIDTH 4
>  #else
>  #define KASAN_TAG_WIDTH 0
>  #endif
> --
> 2.34.1
>