thp: fix page_referenced to modify mapcount/vm_flags only if page is found

When vmscan.c calls page_referenced(), if an anon page was created
before a process forked, rmap will search for it in both of the
processes, even though one of them might have since broken COW.

If the child process mlocks the vma where the COWed page belongs to,
page_referenced() running on the page mapped by the parent would lead to
*vm_flags getting VM_LOCKED set erroneously (leading to the references
on the parent page being ignored and evicting the parent page too
early).

*mapcount would also be decremented by page_referenced_one even if the
page wasn't found by page_check_address.

This also lets pmdp_clear_flush_young_notify() go ahead on a
pmd_trans_splitting() pmd.

We hold the page_table_lock so __split_huge_page_map() must wait the
pmdp_clear_flush_young_notify() to complete before it can modify the
pmd.  The pmd is also still mapped in userland so the young bit may
materialize through a tlb miss before split_huge_page_map runs.

This will provide a more accurate page_referenced() behavior during
split_huge_page().

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Rik van Riel<riel@redhat.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -497,41 +497,51 @@ int page_referenced_one(struct page *page, struct vm_area_struct *vma,
 	struct mm_struct *mm = vma->vm_mm;
 	int referenced = 0;
 
-	/*
-	 * Don't want to elevate referenced for mlocked page that gets this far,
-	 * in order that it progresses to try_to_unmap and is moved to the
-	 * unevictable list.
-	 */
-	if (vma->vm_flags & VM_LOCKED) {
-		*mapcount = 0;	/* break early from loop */
-		*vm_flags |= VM_LOCKED;
-		goto out;
-	}
-
-	/* Pretend the page is referenced if the task has the
-	   swap token and is in the middle of a page fault. */
-	if (mm != current->mm && has_swap_token(mm) &&
-			rwsem_is_locked(&mm->mmap_sem))
-		referenced++;
-
 	if (unlikely(PageTransHuge(page))) {
 		pmd_t *pmd;
 
 		spin_lock(&mm->page_table_lock);
+		/*
+		 * rmap might return false positives; we must filter
+		 * these out using page_check_address_pmd().
+		 */
 		pmd = page_check_address_pmd(page, mm, address,
 					     PAGE_CHECK_ADDRESS_PMD_FLAG);
-		if (pmd && !pmd_trans_splitting(*pmd) &&
-		    pmdp_clear_flush_young_notify(vma, address, pmd))
+		if (!pmd) {
+			spin_unlock(&mm->page_table_lock);
+			goto out;
+		}
+
+		if (vma->vm_flags & VM_LOCKED) {
+			spin_unlock(&mm->page_table_lock);
+			*mapcount = 0;	/* break early from loop */
+			*vm_flags |= VM_LOCKED;
+			goto out;
+		}
+
+		/* go ahead even if the pmd is pmd_trans_splitting() */
+		if (pmdp_clear_flush_young_notify(vma, address, pmd))
 			referenced++;
 		spin_unlock(&mm->page_table_lock);
 	} else {
 		pte_t *pte;
 		spinlock_t *ptl;
 
+		/*
+		 * rmap might return false positives; we must filter
+		 * these out using page_check_address().
+		 */
 		pte = page_check_address(page, mm, address, &ptl, 0);
 		if (!pte)
 			goto out;
 
+		if (vma->vm_flags & VM_LOCKED) {
+			pte_unmap_unlock(pte, ptl);
+			*mapcount = 0;	/* break early from loop */
+			*vm_flags |= VM_LOCKED;
+			goto out;
+		}
+
 		if (ptep_clear_flush_young_notify(vma, address, pte)) {
 			/*
 			 * Don't treat a reference through a sequentially read
@@ -546,6 +556,12 @@ int page_referenced_one(struct page *page, struct vm_area_struct *vma,
 		pte_unmap_unlock(pte, ptl);
 	}
 
+	/* Pretend the page is referenced if the task has the
+	   swap token and is in the middle of a page fault. */
+	if (mm != current->mm && has_swap_token(mm) &&
+			rwsem_is_locked(&mm->mmap_sem))
+		referenced++;
+
 	(*mapcount)--;
 
 	if (referenced)