Re: [PATCH v2] mm: fix KASAN build error due to p*d_populate_kernel()

[Harry Yoo]

On Thu, Aug 21, 2025 at 10:36:12AM -0700, Dave Hansen wrote:
> On 8/21/25 04:57, Harry Yoo wrote:
> > However, {pgd,p4d}_populate_kernel() is defined as a function regardless
> > of the number of page table levels, so the compiler may not optimize
> > them away. In this case, the following linker error occurs:

Hi, thanks for taking a look, Dave!

First of all, this is a fix-up patch of a mm-hotfixes patch series that
fixes a bug (I should have explained that in the changelog) [1].

[1] https://lore.kernel.org/linux-mm/20250818020206.4517-1-harry.yoo@xxxxxxxxxx

I think we can continue discussing it and perhaps do that as part of
a follow-up series, because the current patch series need to be backported
to -stable and your suggestion to improve existing code doesn't require
-stable backports.

Does that sound fine?

> This part of the changelog confused me. I think it's focusing on the
> wrong thing.
> 
> The code that's triggering this is literally:
> 
> >                         pgd_populate(&init_mm, pgd,
> >                                         lm_alias(kasan_early_shadow_p4d));
> 
> It sure _looks_ like it's unconditionally referencing the
> 'kasan_early_shadow_p4d' symbol. I think it's wrong to hide that with
> macro magic and just assume that the macros won't reference it.
> 
> If a symbol isn't being defined, it shouldn't be referenced in C code.:q

A fair point, and that's what KASAN code has been doing for years.

> The right way to do it is to have an #ifdef in a header that avoids
> compiling in the reference to the symbol.

You mean defining some wrapper functions for p*d_populate_kernel() in
KASAN with different implementations based on ifdeffery?

Just to clarify, what should be the exact ifdeffery to cover these cases?
#if CONFIG_PGTABLE_LEVELS == 4 and 5, or
#ifdef __PAGETABLE_P4D_FOLDED and __PAGETABLE_PUD_FOLDED ?

I have no strong opinion on this, let's hear what KASAN folks think.

> But just changing the 'static inline' to a #define seems like a fragile
> hack to me.

At least that's what KASAN has relied on p*d_populate() to do...

-- 
Cheers,
Harry / Hyeonggon