mm: simplify try_to_unmap_one()

SWAP_MLOCK mean "We marked the page as PG_MLOCK, please move it to
unevictable-lru". So, following code is easy confusable.

        if (vma->vm_flags & VM_LOCKED) {
                ret = SWAP_MLOCK;
                goto out_unmap;
        }

Plus, if the VMA doesn't have VM_LOCKED, We don't need to check
the needed of calling mlock_vma_page().

Also, add some commentary to try_to_unmap_one().
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-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

@@ -789,10 +789,9 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 	 * skipped over this mm) then we should reactivate it.
 	 */
 	if (!(flags & TTU_IGNORE_MLOCK)) {
-		if (vma->vm_flags & VM_LOCKED) {
-			ret = SWAP_MLOCK;
-			goto out_unmap;
-		}
+		if (vma->vm_flags & VM_LOCKED)
+			goto out_mlock;
+
 		if (TTU_ACTION(flags) == TTU_MUNLOCK)
 			goto out_unmap;
 	}
@@ -865,9 +864,21 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
+out:
+	return ret;
+
+out_mlock:
+	pte_unmap_unlock(pte, ptl);
 
-	if (ret == SWAP_MLOCK) {
-		ret = SWAP_AGAIN;
+
+	/*
+	 * We need mmap_sem locking, Otherwise VM_LOCKED check makes
+	 * unstable result and race. Plus, We can't wait here because
+	 * we now hold anon_vma->lock or mapping->i_mmap_lock.
+	 * if trylock failed, the page remain in evictable lru and later
+	 * vmscan could retry to move the page to unevictable lru if the
+	 * page is actually mlocked.
+	 */
 	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
 		if (vma->vm_flags & VM_LOCKED) {
 			mlock_vma_page(page);
@@ -875,8 +886,6 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 		}
 		up_read(&vma->vm_mm->mmap_sem);
 	}
-	}
-out:
 	return ret;
 }