[PATCH 2/3] mm: Allow arches to provide ptep_get()

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Since commit 2ab3a0a02905 ("READ_ONCE: Enforce atomicity for
{READ,WRITE}_ONCE() memory accesses"), it is not possible anymore
to use READ_ONCE() to access complex page table entries like
the one defined for powerpc 8xx with 16k size pages.

Define a ptep_get() helper that architectures can override instead
of performing a READ_ONCE() on the page table entry pointer.

Fixes: 2ab3a0a02905 ("READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses")
Cc: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
 include/asm-generic/hugetlb.h | 2 +-
 include/linux/pgtable.h       | 7 +++++++
 mm/gup.c                      | 2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/hugetlb.h b/include/asm-generic/hugetlb.h
index 40f85decc2ee..8e1e6244a89d 100644
--- a/include/asm-generic/hugetlb.h
+++ b/include/asm-generic/hugetlb.h
@@ -122,7 +122,7 @@ static inline int huge_ptep_set_access_flags(struct vm_area_struct *vma,
 #ifndef __HAVE_ARCH_HUGE_PTEP_GET
 static inline pte_t huge_ptep_get(pte_t *ptep)
 {
-	return READ_ONCE(*ptep);
+	return ptep_get(ptep);
 }
 #endif
 
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 32b6c52d41b9..56c1e8eb7bb0 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -249,6 +249,13 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 }
 #endif
 
+#ifndef __HAVE_ARCH_PTEP_GET
+static inline pte_t ptep_get(pte_t *ptep)
+{
+	return READ_ONCE(*ptep);
+}
+#endif
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 #ifndef __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
 static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
diff --git a/mm/gup.c b/mm/gup.c
index 761df4944ef5..6f47697f8fb0 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2196,7 +2196,7 @@ static inline pte_t gup_get_pte(pte_t *ptep)
  */
 static inline pte_t gup_get_pte(pte_t *ptep)
 {
-	return READ_ONCE(*ptep);
+	return ptep_get(ptep);
 }
 #endif /* CONFIG_GUP_GET_PTE_LOW_HIGH */
 
-- 
2.25.0