Via commit 4b3ef9da ("mm/swap: split swap cache into 64MB trunks") on,
after swapoff, the address_space associated with the swap device will be
freed.  So swap_address_space() users which touch the address_space need
some kind of mechanism to prevent the address_space from being freed
during accessing.

When mincore process unmapped range for swapped shmem pages, it doesn't
hold the lock to prevent swap device from being swapoff.  So the following
race is possible,

CPU1					CPU2
do_mincore()				swapoff()
  walk_page_range()
    mincore_unmapped_range()
      __mincore_unmapped_range
        mincore_page
	  as = swap_address_space()
          ...				  exit_swap_address_space()
          ...				    kvfree(spaces)
	  find_get_page(as)

The address space may be accessed after being freed.

To fix the race, get_swap_device()/put_swap_device() is used to enclose
find_get_page() to check whether the swap entry is valid and prevent the
swap device from being swapoff during accessing.

Link: http://lkml.kernel.org/r/20180313012036.1597-1-ying.huang@intel.com
Fixes: 4b3ef9da

 ("mm/swap: split swap cache into 64MB trunks")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>