linux-kernel - Re: [PATCH v2 00/12] mm/kasan: make kasan=on|off work for all three modes

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Message-ID: <CA+fCnZdKy-AQr+L3w=gfaw9EnFvKd0Gz4LtAZciYDP_SiWrL2A@mail.gmail.com>
Date: Tue, 12 Aug 2025 19:14:52 +0200
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, sj@...nel.org, lorenzo.stoakes@...cle.com, 
	elver@...gle.com, snovitoll@...il.com
Subject: Re: [PATCH v2 00/12] mm/kasan: make kasan=on|off work for all three modes

On Tue, Aug 12, 2025 at 6:57 PM Andrey Konovalov <andreyknvl@...il.com> wrote:
>
> On Tue, Aug 12, 2025 at 2:49 PM 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
> > for kasan shadow while in fact it's meaningless to have kasan in kdump
> > kernel.
> >
> > 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.
>
> Hi Baoquan,
>
> Could you clarify what are you trying to achieve by disabling
> Generic/SW_TAGS KASAN via command-line? Do you want not to see any
> KASAN reports produced? Or gain back the performance?
>
> Because for the no reports goal, it would be much easier to add a
> command-line parameter to silent the reports.
>
> And the performance goal can only be partially achieved, as you cannot
> remove the compiler instrumentation without rebuilding the kernel.
> (What are the boot times for KASAN_GENERIC=n vs KASAN_GENERIC=y +
> kasan=off vs KASAN_GENERIC=y btw?)
>
> Thank you!

Ah, you don't want the shadow memory for kdump, sorry, I somehow missed that.

I'm not familiar with the internals of kdump, but would it be
possible/reasonable to teach kdump to ignore the KASAN shadow region?