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>

diff --git a/mm/rmap.c b/mm/rmap.c
index c81bedd..98135db 100644 (file)
--- a/mm/rmap.c
+++ b/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;
        }
@@ -865,18 +864,28 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 
 out_unmap:
        pte_unmap_unlock(pte, ptl);
+out:
+       return ret;
 
-       if (ret == SWAP_MLOCK) {
-               ret = SWAP_AGAIN;
-               if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
-                       if (vma->vm_flags & VM_LOCKED) {
-                               mlock_vma_page(page);
-                               ret = SWAP_MLOCK;
-                       }
-                       up_read(&vma->vm_mm->mmap_sem);
+out_mlock:
+       pte_unmap_unlock(pte, ptl);
+
+
+       /*
+        * 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);
+                       ret = SWAP_MLOCK;
                }
+               up_read(&vma->vm_mm->mmap_sem);
        }
-out:
        return ret;
 }