mm: fix races between address_space dereference and free in page_evicatable

When page_mapping() is called and the mapping is dereferenced in
page_evicatable() through shrink_active_list(), it is possible for the
inode to be truncated and the embedded address space to be freed at the
same time.  This may lead to the following race.

CPU1                                                CPU2

truncate(inode)                                     shrink_active_list()
  ...                                                 page_evictable(page)
  truncate_inode_page(mapping, page);
    delete_from_page_cache(page)
      spin_lock_irqsave(&mapping->tree_lock, flags);
        __delete_from_page_cache(page, NULL)
          page_cache_tree_delete(..)
            ...                                         mapping = page_mapping(page);
            page->mapping = NULL;
            ...
      spin_unlock_irqrestore(&mapping->tree_lock, flags);
      page_cache_free_page(mapping, page)
        put_page(page)
          if (put_page_testzero(page)) -> false
- inode now has no pages and can be freed including embedded address_space

                                                        mapping_unevictable(mapping)
							  test_bit(AS_UNEVICTABLE, &mapping->flags);
- we've dereferenced mapping which is potentially already free.

Similar race exists between swap cache freeing and page_evicatable()
too.

The address_space in inode and swap cache will be freed after a RCU
grace period.  So the races are fixed via enclosing the page_mapping()
and address_space usage in rcu_read_lock/unlock().  Some comments are
added in code to make it clear what is protected by the RCU read lock.

Link: http://lkml.kernel.org/r/20180212081227.1940-1-ying.huang@intel.com


Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Minchan Kim <minchan@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>