2019-04-23 15:18:45

by Laurent Dufour

Subject: [PATCH] powerpc/mm: Comment arch_unmap()

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 <[email protected]>
Cc: Michael Ellerman <[email protected]>
Signed-off-by: Laurent Dufour <[email protected]>
---
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)
+ * 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;
}
--
2.21.0

2019-04-23 16:48:52

by Laurent Dufour

Subject: Re: [PATCH] powerpc/mm: Comment arch_unmap()

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 <[email protected]>
> Cc: Michael Ellerman <[email protected]>
> Signed-off-by: Laurent Dufour <[email protected]>
> ---
> 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;
> }
>