linux-kernel - Re: [PATCH] mm: alloc_tag: change the KASAN_TAG_WIDTH for HW

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Message-Id: <20250427172551.fbf81f14d4e6563c13007a65@linux-foundation.org>
Date: Sun, 27 Apr 2025 17:25:51 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Guilherme Giacomo Simoes <trintaeoitogc@...il.com>
Cc: pasha.tatashin@...een.com, surenb@...gle.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Andrey Konovalov <andreyknvl@...il.com>
Subject: Re: [PATCH] mm: alloc_tag: change the KASAN_TAG_WIDTH for HW_TAGS

On Sat, 26 Apr 2025 15:08:37 -0300 Guilherme Giacomo Simoes <trintaeoitogc@...il.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.