Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot_stage

[Ingo Molnar <mingo@kernel.org>]

* Thomas Garnier <thgarnie@google.com> wrote:

> >  static inline void setup_fixmap_gdt(int cpu)
> >  {
> >         __set_fixmap(get_cpu_gdt_ro_index(cpu),
> > -                    __pa(get_cpu_gdt_rw(cpu)), pg_fixmap_gdt_flags);
> > +                    slow_virt_to_phys(get_cpu_gdt_rw(cpu)),
> > +                    pg_fixmap_gdt_flags);
> >  }
> >
> >  /* Load the original GDT from the per-cpu structure */
> >
> > This makes UP boot for me, but SMP (2 cpus) is still busted.
> 
> This change fixed boot for me:
> 
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index b65155cc3760..4e30707d9f9a 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -104,7 +104,9 @@ enum fixed_addresses {
>         FIX_GDT_REMAP_BEGIN,
>         FIX_GDT_REMAP_END = FIX_GDT_REMAP_BEGIN + NR_CPUS - 1,
> 
> -       __end_of_permanent_fixed_addresses,
> +       __end_of_permanent_fixed_addresses =
> +               (FIX_GDT_REMAP_END + PTRS_PER_PTE - 1) &
> +               -PTRS_PER_PTE,
> 
> Just ensure PKMAP_BASE & FIX_WP_TEST are on a different PMD.
> 
> I don't think that the right fix but it might help understand the
> exact root cause.

Could this be related to the permission bits in the PMD itself getting out of sync 
with the PTEs? WP test marks a page writable/unwritable, and maybe we mess up the 
restoration. If they are on separate PMDs then this is worked around because the 
fixmap GDT is on a separate PMD.

Thanks,

	Ingo