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

[Baoquan He <bhe@redhat.com>]

On 08/12/25 at 07:14pm, Andrey Konovalov wrote:
> On Tue, Aug 12, 2025 at 6:57 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
> >
> > On Tue, Aug 12, 2025 at 2:49 PM Baoquan He <bhe@redhat.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?)
> >

Thanks a lot for checking this.

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

Yeah, for kdump kernel, the shadow is a heavy burden, and most
importantly kasan is useless for kdump. We don't want to capture a
kernel memory bug through kdump kernel running becuase kdump is a
debugging mechanism.

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

Yes, we can teach kdump to do that. Then people may hate those conditional
check "if (is_kdump_kernel())" being added in kasan code. E.g even
though we skip kasan_init(), we still need to check is_kdump_kernel()
in kasan_populate_vmalloc(), right? 

Combined with the existing kasan_arch_is_ready(), it will make kasan code
ugly. I planned to add kasan_enabled() via static key
kasan_flag_enabled, then it can also easily remove kasan_arch_is_ready()
cleanly.