Re: [PATCH V4 8/8] riscv: Add config of thread stack size

[Guo Ren]

On Mon, Sep 12, 2022 at 4:25 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Mon, Sep 12, 2022, at 6:14 AM, Guo Ren wrote:
> > On Mon, Sep 12, 2022 at 2:40 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >> On Sun, Sep 11, 2022, at 1:35 AM, Guo Ren wrote:
> >> > On Sun, Sep 11, 2022 at 12:07 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >> >>
> >> >> That sounds like a really bad idea, why would you want to risk
> >> >> using such a small stack without CONFIG_VMAP_STACK?
> >> >>
> >> >> Are you worried about increased memory usage or something else?
> >> > The requirement is from [1], and I think disabling CONFIG_VMAP_STACK
> >> > would be the last step after serious testing.
> >>
> >> I still don't see why you need to turn off VMAP_STACK at all
> >> if it works. The only downside I can see with using VMAP_STACK
> >> on a 64-bit system is that it may expose bugs with device
> >> drivers that do DMA to stack data. Once you have tested the
> >> system successfully, you can also assume that you have no such
> >> drivers.
> > 1st, VMAP_STACK could be enabled&disabled in arch/Kconfig. If we don't
> > force users to enable VMAP_STACK, why couldn't user adjust
> > THREAD_SIZE?
>
> Turning off VMAP_STACK is harmless and may help debug issues
> with VMAP_STACK itself. It's also required on architectures
> that don't have KASAN_VMALLOC or something else that conflicts
> with it.
>
> Changing THREAD_SIZE is also fine, as long as VMAP_STACK catches
> the inevitable overflows. I would not object to having an
> option that allows setting the stack size larger than the
> default without VMAP_STACK, as long as setting it lower requires
> using VMAP_STACK. That would however add a lot more complexity
> and probably doesn't do what you want either.
Thx for the detailed clarification, I agree with the point. I've put
an EXPERT on config.

>
> > 2nd, VMAP_STACK is not free, we still need 1KB shadow_stack.
> > The EXPERT is enough for your concern.
>
> It's actually more than the 1KB: you need both 1KB of shadow
> stack and 4KB per CPU for the actual overflow_stack. If you
> are micro-optimizing at this level, then a possible option
> may be to change the handle_kernel_stack_overflow() function
> to not preserve the task stack and just panic() without
> showing the backtrace. That way you don't see which code
> caused the issue, but at least you avoid corrupting random
> data.
Thx for the detailed explanation, the handle_kernel_stack_overflow()
is a novel idea, which I will consider later.

>
>      Arnd



--
Best Regards
 Guo Ren