[PATCH/RFC 2/7] mm: replace page table access via ACCESS_ONCE with barriers

[Christian Borntraeger]

ACCESS_ONCE does not work reliably on non-scalar types. For
example gcc 4.6 and 4.7 might remove the volatile tag for such
accesses during the SRA (scalar replacement of aggregates) step
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)

Let's change the code to access the page table elements with
a barrier afterwards.

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
 mm/gup.c    | 4 +++-
 mm/memory.c | 3 ++-
 mm/rmap.c   | 3 ++-
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index cd62c8c..e44af3c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -917,7 +917,9 @@ static int gup_pud_range(pgd_t *pgdp, unsigned long addr, unsigned long end,
 
 	pudp = pud_offset(pgdp, addr);
 	do {
-		pud_t pud = ACCESS_ONCE(*pudp);
+		pud_t pud = *pudp;
+
+		barrier();
 
 		next = pud_addr_end(addr, end);
 		if (pud_none(pud))
diff --git a/mm/memory.c b/mm/memory.c
index 3e50383..d982e35 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3202,7 +3202,8 @@ static int handle_pte_fault(struct mm_struct *mm,
 	pte_t entry;
 	spinlock_t *ptl;
 
-	entry = ACCESS_ONCE(*pte);
+	entry = *pte;
+	barrier();
 	if (!pte_present(entry)) {
 		if (pte_none(entry)) {
 			if (vma->vm_ops) {
diff --git a/mm/rmap.c b/mm/rmap.c
index 19886fb..1e54274 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -581,7 +581,8 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
 	 * without holding anon_vma lock for write.  So when looking for a
 	 * genuine pmde (in which to find pte), test present and !THP together.
 	 */
-	pmde = ACCESS_ONCE(*pmd);
+	pmde = *pmd;
+	barrier();
 	if (!pmd_present(pmde) || pmd_trans_huge(pmde))
 		pmd = NULL;
 out:
-- 
1.9.3

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