userfaultfd: shmem: avoid a lockup resulting from corrupted page->flags

Use the non atomic version of __SetPageUptodate while the page is still
private and not visible to lookup operations.  Using the non atomic
version after the page is already visible to lookups is unsafe as there
would be concurrent lock_page operation modifying the page->flags while
it runs.

This solves a lockup in find_lock_entry with the userfaultfd_shmem
selftest.

  userfaultfd_shm D14296   691      1 0x00000004
  Call Trace:
   schedule+0x3d/0x90
   schedule_timeout+0x228/0x420
   io_schedule_timeout+0xa4/0x110
   __lock_page+0x12d/0x170
   find_lock_entry+0xa4/0x190
   shmem_getpage_gfp+0xb9/0xc30
   shmem_fault+0x70/0x1c0
   __do_fault+0x21/0x150
   handle_mm_fault+0xec9/0x1490
   __do_page_fault+0x20d/0x520
   trace_do_page_fault+0x61/0x270
   do_async_page_fault+0x19/0x80
   async_page_fault+0x25/0x30

Link: http://lkml.kernel.org/r/20170116180408.12184-2-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -2248,6 +2248,7 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	VM_BUG_ON(PageLocked(page) || PageSwapBacked(page));
 	__SetPageLocked(page);
 	__SetPageSwapBacked(page);
+	__SetPageUptodate(page);
 
 	ret = mem_cgroup_try_charge(page, dst_mm, gfp, &memcg, false);
 	if (ret)
@@ -2272,8 +2273,6 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	if (!pte_none(*dst_pte))
 		goto out_release_uncharge_unlock;
 
-	__SetPageUptodate(page);
-
 	lru_cache_add_anon(page);
 
 	spin_lock(&info->lock);