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

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

[Guilherme Giacomo Simoes]

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.

Suggested-by: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Guilherme Giacomo Simoes <trintaeoitogc@gmail.com>
---
 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

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

[Andrew Morton]

On Sat, 26 Apr 2025 15:08:37 -0300 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.

Why?

I assume the effect is to save a few flags bits under some
circumstances?  But that's just me guessing - please fully describe the
motivation and effects within changelogs.

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

[Guilherme Giacomo Simoes]

Andrew Morton <akpm@linux-foundation.org> 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.
> 
> Why?
> 
> I assume the effect is to save a few flags bits under some
> circumstances?  But that's just me guessing - please fully describe the
> motivation and effects within changelogs.
Ok, I will change the commit message with the "why", and send a v2.

Thanks.