mm: thp: set the accessed flag for old pages on access fault

On x86 memory accesses to pages without the ACCESSED flag set result in
the ACCESSED flag being set automatically.  With the ARM architecture a
page access fault is raised instead (and it will continue to be raised
until the ACCESSED flag is set for the appropriate PTE/PMD).

For normal memory pages, handle_pte_fault will call pte_mkyoung
(effectively setting the ACCESSED flag).  For transparent huge pages,
pmd_mkyoung will only be called for a write fault.

This patch ensures that faults on transparent hugepages which do not
result in a CoW update the access flags for the faulting pmd.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ni zhan Chen <nizhan.chen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -784,6 +784,28 @@ out:
 	return ret;
 }
 
+void huge_pmd_set_accessed(struct mm_struct *mm,
+			   struct vm_area_struct *vma,
+			   unsigned long address,
+			   pmd_t *pmd, pmd_t orig_pmd,
+			   int dirty)
+{
+	pmd_t entry;
+	unsigned long haddr;
+
+	spin_lock(&mm->page_table_lock);
+	if (unlikely(!pmd_same(*pmd, orig_pmd)))
+		goto unlock;
+
+	entry = pmd_mkyoung(orig_pmd);
+	haddr = address & HPAGE_PMD_MASK;
+	if (pmdp_set_access_flags(vma, haddr, pmd, entry, dirty))
+		update_mmu_cache_pmd(vma, address, pmd);
+
+unlock:
+	spin_unlock(&mm->page_table_lock);
+}
+
 static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
 					struct vm_area_struct *vma,
 					unsigned long address,

mm/memory.c

@@ -3537,8 +3537,9 @@ retry:
 
 		barrier();
 		if (pmd_trans_huge(orig_pmd)) {
-			if (flags & FAULT_FLAG_WRITE &&
-			    !pmd_write(orig_pmd) &&
+			unsigned int dirty = flags & FAULT_FLAG_WRITE;
+
+			if (dirty && !pmd_write(orig_pmd) &&
 			    !pmd_trans_splitting(orig_pmd)) {
 				ret = do_huge_pmd_wp_page(mm, vma, address, pmd,
 							  orig_pmd);
@@ -3550,6 +3551,9 @@ retry:
 				if (unlikely(ret & VM_FAULT_OOM))
 					goto retry;
 				return ret;
+			} else {
+				huge_pmd_set_accessed(mm, vma, address, pmd,
+						      orig_pmd, dirty);
 			}
 			return 0;
 		}