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>

1 files changed, 22 insertions, 13 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index c81bedd7d52..98135dbd25b 100644
--- 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;
 }