Use the helper for the checks.  Rename "check_mapping" into "zap_mapping"
because "check_mapping" looks like a bool but in fact it stores the mapping
itself.  When it's set, we check the mapping (it must be non-NULL).  When it's
cleared we skip the check, which works like the old way.

Move the duplicated comments to the helper too.

Reviewed-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
  include/linux/mm.h | 16 +++++++++++++++-
  mm/memory.c        | 29 ++++++-----------------------
  2 files changed, 21 insertions(+), 24 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d1126f731221..ed44f31615d9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -1721,10 +1721,24 @@ extern void user_shm_unlock(size_t, struct ucounts *);
   * Parameter block passed down to zap_pte_range in exceptional cases.
   */
  struct zap_details {
-	struct address_space *check_mapping;	/* Check page->mapping if set */
+	struct address_space *zap_mapping;	/* Check page->mapping if set */
  	struct page *single_page;		/* Locked page to be unmapped */
  };
  
+/*
+ * We set details->zap_mappings when we want to unmap shared but keep private
+ * pages. Return true if skip zapping this page, false otherwise.
+ */
+static inline bool
+zap_skip_check_mapping(struct zap_details *details, struct page *page)