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@oracle.com
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...