[PATCH] powerpc/mm: Comment arch_unmap()

[Laurent Dufour <ldufour@linux.ibm.com>]

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@linutronix.de>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Laurent Dufour <ldufour@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)
+	 *  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