[PATCH] powerpc/mm: Comment arch_unmap()

[Laurent Dufour]

Le 23/04/2019 à 17:17, Laurent Dufour a écrit :
> During a different patch review, the check in arch_munmap() was found
> spucious due the lake of explanation.
> 
> Adding a comment to clarify the test.
> 
> Suggested-by: Thomas Gleixner <tglx at linutronix.de>
> Cc: Michael Ellerman <mpe at ellerman.id.au>
> Signed-off-by: Laurent Dufour <ldufour at linux.ibm.com>
> ---
>   arch/powerpc/include/asm/mmu_context.h | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> index 6ee8195a2ffb..638f481b3c9f 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -240,6 +240,17 @@ static inline void arch_unmap(struct mm_struct *mm,
>   			      struct vm_area_struct *vma,
>   			      unsigned long start, unsigned long end)
>   {
> +	/*
> +	 * There are 2 assumptions here:
> +	 *  1. the VDSO is one page size (guaranteed by vdso_data_store)

Indeed this is not true, only the descriptor is one page size.
This makes that test not handling all the cases, especially if a upper 
part of the VDSO is unmap (start > mm->context.vdso_base).

I'll sent a new fix asap.

> +	 *  2. 'start' and 'end' are page aligned (guaranteed by the caller)
> +	 * The test is wrote in a way to handle unmap operation surrounding the
> +	 * VDSO area like:
> +	 *            | VDSO |
> +	 *    ^ start          ^ end
> +	 * The test also covers the munmap() operation done to the exact VDSO's
> +	 * boundaries.
> +	 */
>   	if (start <= mm->context.vdso_base && mm->context.vdso_base < end)
>   		mm->context.vdso_base = 0;
>   }
>