arm64: Fix copy-on-write referencing in HugeTLB

set_pte_at(.) will set or unset the PTE_RDONLY hardware bit before
writing the entry to the table.

This can cause problems with the copy-on-write logic in hugetlb_cow:
 *) hugetlb_cow(.) called to handle a write fault on read only pte,
 *) Before the copy-on-write updates the new page table a call is
    made to pte_same(huge_ptep_get(ptep), pte)), to check for a race,
 *) Because set_pte_at(.) changed the pte, *ptep != pte, and the
    hugetlb_cow(.) code erroneously assumes that it lost the race,
 *) The new page is subsequently freed without being used.

On arm64 this problem only becomes apparent when we apply:
67961f9d mm/hugetlb: fix huge page reserve accounting for private
mappings

When one runs the libhugetlbfs test suite, there are allocation errors
and hugetlbfs pages become erroneously locked in memory as reserved.
(There is a high HugePages_Rsvd: count).

In this patch we introduce pte_same which ignores the PTE_RDONLY bit,
allowing for the libhugetlbfs test suite to pass as expected and
without leaking any reserved HugeTLB pages.
Reported-by: Huang Shijie <shijie.huang@arm.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pgtable.h

@@ -224,6 +224,23 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 	set_pte(ptep, pte);
 }
 
+#define __HAVE_ARCH_PTE_SAME
+static inline int pte_same(pte_t pte_a, pte_t pte_b)
+{
+	pteval_t lhs, rhs;
+
+	lhs = pte_val(pte_a);
+	rhs = pte_val(pte_b);
+
+	if (pte_present(pte_a))
+		lhs &= ~PTE_RDONLY;
+
+	if (pte_present(pte_b))
+		rhs &= ~PTE_RDONLY;
+
+	return (lhs == rhs);
+}
+
 /*
  * Huge pte definitions.
  */