userfaultfd: shmem: lock the page before adding it to pagecache

A VM_BUG_ON triggered on the shmem selftest.

Link: http://lkml.kernel.org/r/20161216144821.5183-36-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -2245,6 +2245,10 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
 		*pagep = NULL;
 	}
 
+	VM_BUG_ON(PageLocked(page) || PageSwapBacked(page));
+	__SetPageLocked(page);
+	__SetPageSwapBacked(page);
+
 	ret = mem_cgroup_try_charge(page, dst_mm, gfp, &memcg, false);
 	if (ret)
 		goto out_release;
@@ -2294,6 +2298,7 @@ out_release_uncharge_unlock:
 out_release_uncharge:
 	mem_cgroup_cancel_charge(page, memcg, false);
 out_release:
+	unlock_page(page);
 	put_page(page);
 out_dec_used_blocks:
 	if (sbinfo->max_blocks)