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

[Baoquan He <bhe@redhat.com>]

On 08/16/25 at 06:50am, Andrey Konovalov wrote:
> On Thu, Aug 14, 2025 at 10:56 AM Baoquan He <bhe@redhat.com> wrote:
> >
> > Ah, I got what you mean. We probably are saying different things.
> >
> > In order to record memory content of a corrupted kernel, we need reserve
> > a memory region during bootup of a normal kernel (usually called 1st
> > kernel) via kernel parameter crashkernel=nMB in advance. Then load
> > kernel into the crashkernel memory region, that means the region is not
> > usable for 1st kernel. When 1st kernel collapsed, we stop the 1st kernel
> > cpu/irq and warmly switch to the loaded kernel in the crashkernel memory
> > region (usually called kdump kernel). In kdump kernel, it boots up and
> > enable necessary features to read out the 1st kernel's memory content,
> > we usually use user space tool like makeudmpfile to filter out unwanted
> > memory content.
> >
> > So this patchset intends to disable KASAN to decrease the crashkernel
> > meomry value because crashkernel is not usable for 1st kernel. As for
> > shadow memory of 1st kernel, we need recognize it and filter it away
> > in makedumpfile.
> 
> Ah, I see, thank you for the explanation!
> 
> So kdump kernel runs with the amount of RAM specified by crashkernel=.
> And KASAN's shadow memory increases RAM usage, which means
> crashkernel= needs to be set to a higher value for KASAN kernels. Is
> my understanding of the problem correct?

Yeah, you are quite right.

When I tested it, on x86_64 and arm64, usually I set crashkernel=256M
and it's sufficient. However, when KASAN is enabled and generic mode is
taken, I need set crashkernel=768M to make vmcore dumping succeed. In
kdump kernel, read_vmcore() uses ioremap to map the old memory of
collapsed kernel for reading out, those vmalloc-ed areas are lazily
freed and cause more shadow memory than what we usually think shadow
memory only costs 1/8 of physical RAM.