Message-ID: <CA+fCnZcVV5=AJUNfy6G2T-UZCbAL=7NivmWkBr6LMSnzzTZ8Kg@mail.gmail.com>
Date: Thu, 4 Dec 2025 17:38:49 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: linux-mm@...ck.org, ryabinin.a.a@...il.com, glider@...gle.com, 
	dvyukov@...gle.com, vincenzo.frascino@....com, akpm@...ux-foundation.org, 
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, 
	kexec@...ts.infradead.org, elver@...gle.com, sj@...nel.org, 
	lorenzo.stoakes@...cle.com, snovitoll@...il.com, christophe.leroy@...roup.eu
Subject: Re: [PATCH v4 00/12] mm/kasan: make kasan=on|off work for all three modes

On Fri, Nov 28, 2025 at 4:33 AM Baoquan He <bhe@...hat.com> wrote:
>
> Currently only hw_tags mode of kasan can be enabled or disabled with
> kernel parameter kasan=on|off for built kernel. For kasan generic and
> sw_tags mode, there's no way to disable them once kernel is built.
>
> This is not convenient sometime, e.g in system kdump is configured.
> When the 1st kernel has KASAN enabled and crash triggered to switch to
> kdump kernel, the generic or sw_tags mode will cost much extra memory
> while in fact it's meaningless to have kasan in kdump kernel
>
> There are two parts of big amount of memory requiring for kasan enabed
> kernel. One is the direct memory mapping shadow of kasan, which is 1/8
> of system RAM in generic mode and 1/16 of system RAM in sw_tags mode;
> the other is the shadow meomry for vmalloc which causes big meomry
> usage in kdump kernel because of lazy vmap freeing. By introducing
> "kasan=off|on", if we specify 'kasan=off', the former is avoided by skipping
> the kasan_init(), and the latter is avoided by not building the vmalloc
> shadow for vmalloc.
>
> So this patchset moves the kasan=on|off out of hw_tags scope and into
> common code to make it visible in generic and sw_tags mode too. Then we
> can add kasan=off in kdump kernel to reduce the unneeded meomry cost for
> kasan.
>
> Testing:
> ========
> - Testing on x86_64 and arm64 for generic mode passed when kasan=on or
>   kasan=off.
>
> - Testing on arm64 with sw_tags mode passed when kasan=off is set. But
>   when I tried to test sw_tags on arm64, the system bootup failed. It's
>   not introduced by my patchset, the original code has the bug. I have
>   reported it to upstream.
>   - System is broken in KASAN sw_tags mode during bootup
>     - https://lore.kernel.org/all/aSXKqJTkZPNskFop@MiWiFi-R3L-srv/T/#u

This will hopefully be fixed soon, so you'll be able to test.

>
> - Haven't found hardware to test hw_tags. If anybody has the system,
>   please help take a test.

You don't need hardware to run the HW_TAGS mode, just pass -machine
virt,mte=on to QEMU.

I also wonder if we should keep this kasan=off functionality
conservative and limit it to x86 and arm64 (since these are the only
two tested architectures).

>
> Changelog:
> ====
> v3->v4:
> - Rebase code to the latest linux-next/master to make the whole patchset
>   set on top of
>   [PATCH 0/2] kasan: cleanups for kasan_enabled() checks
>   [PATCH v6 0/2] kasan: unify kasan_enabled() and remove arch-specific implementations

Note that are also:

[PATCH 1/2] kasan: remove __kasan_save_free_info wrapper
[PATCH 2/2] kasan: cleanup of kasan_enabled() checks

But I don't know if there will be any conflicts with these.

>
> v2->v3:
> - Fix a building error on UML ARCH when CONFIG_KASAN is not set. The
>   change of fixing is appended into patch patch 11. This is reported
>   by LKP, thanks to them.
>
> v1->v2:
> - Add __ro_after_init for kasan_arg_disabled, and remove redundant blank
>   lines in mm/kasan/common.c. Thanks to Marco.
> - Fix a code bug in <linux/kasan-enabled.h> when CONFIG_KASAN is unset,
>   this is found out by SeongJae and Lorenzo, and also reported by LKP
>   report, thanks to them.
> - Add a missing kasan_enabled() checking in kasan_report(). This will
>   cause below KASAN report info even though kasan=off is set:
>      ==================================================================
>      BUG: KASAN: stack-out-of-bounds in tick_program_event+0x130/0x150
>      Read of size 4 at addr ffff00005f747778 by task swapper/0/1
>
>      CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.16.0+ #8 PREEMPT(voluntary)
>      Hardware name: GIGABYTE R272-P30-JG/MP32-AR0-JG, BIOS F31n (SCP: 2.10.20220810) 09/30/2022
>      Call trace:
>       show_stack+0x30/0x90 (C)
>       dump_stack_lvl+0x7c/0xa0
>       print_address_description.constprop.0+0x90/0x310
>       print_report+0x104/0x1f0
>       kasan_report+0xc8/0x110
>       __asan_report_load4_noabort+0x20/0x30
>       tick_program_event+0x130/0x150
>       ......snip...
>      ==================================================================
>
> - Add jump_label_init() calling before kasan_init() in setup_arch() in these
>   architectures: xtensa, arm. Because they currenly rely on
>   jump_label_init() in main() which is a little late. Then the early static
>   key kasan_flag_enabled in kasan_init() won't work.
>
> - In UML architecture, change to enable kasan_flag_enabled in arch_mm_preinit()
>   because kasan_init() is enabled before main(), there's no chance to operate
>   on static key in kasan_init().
>
> Baoquan He (12):
>   mm/kasan: add conditional checks in functions to return directly if
>     kasan is disabled
>   mm/kasan: move kasan= code to common place
>   mm/kasan/sw_tags: don't initialize kasan if it's disabled
>   arch/arm: don't initialize kasan if it's disabled
>   arch/arm64: don't initialize kasan if it's disabled
>   arch/loongarch: don't initialize kasan if it's disabled
>   arch/powerpc: don't initialize kasan if it's disabled
>   arch/riscv: don't initialize kasan if it's disabled
>   arch/x86: don't initialize kasan if it's disabled
>   arch/xtensa: don't initialize kasan if it's disabled
>   arch/um: don't initialize kasan if it's disabled
>   mm/kasan: make kasan=on|off take effect for all three modes
>
>  arch/arm/kernel/setup.c                |  6 ++++++
>  arch/arm/mm/kasan_init.c               |  2 ++
>  arch/arm64/mm/kasan_init.c             |  6 ++++++
>  arch/loongarch/mm/kasan_init.c         |  2 ++
>  arch/powerpc/mm/kasan/init_32.c        |  5 ++++-
>  arch/powerpc/mm/kasan/init_book3e_64.c |  3 +++
>  arch/powerpc/mm/kasan/init_book3s_64.c |  3 +++
>  arch/riscv/mm/kasan_init.c             |  3 +++
>  arch/um/kernel/mem.c                   |  5 ++++-
>  arch/x86/mm/kasan_init_64.c            |  3 +++
>  arch/xtensa/kernel/setup.c             |  1 +
>  arch/xtensa/mm/kasan_init.c            |  3 +++
>  include/linux/kasan-enabled.h          |  6 ++++--
>  mm/kasan/common.c                      | 20 ++++++++++++++++--
>  mm/kasan/generic.c                     | 17 ++++++++++++++--
>  mm/kasan/hw_tags.c                     | 28 ++------------------------
>  mm/kasan/init.c                        |  6 ++++++
>  mm/kasan/quarantine.c                  |  3 +++
>  mm/kasan/report.c                      |  4 +++-
>  mm/kasan/shadow.c                      | 11 +++++++++-
>  mm/kasan/sw_tags.c                     |  6 ++++++
>  21 files changed, 107 insertions(+), 36 deletions(-)

One part that's still missing is a Documentation change.


>
> --
> 2.41.0
>

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