mm, THP, swap: don't allocate huge cluster for file backed swap device

It's hard to write a whole transparent huge page (THP) to a file backed
swap device during swapping out and the file backed swap device isn't
very popular.  So the huge cluster allocation for the file backed swap
device is disabled.

Link: http://lkml.kernel.org/r/20170724051840.2309-5-ying.huang@intel.comSigned-off-by: N"Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Ross Zwisler <ross.zwisler@intel.com> [for brd.c, zram_drv.c, pmem.c]
Cc: Vishal L Verma <vishal.l.verma@intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -948,9 +948,10 @@ int get_swap_pages(int n_goal, bool cluster, swp_entry_t swp_entries[])
 			spin_unlock(&si->lock);
 			goto nextsi;
 		}
-		if (cluster)
-			n_ret = swap_alloc_cluster(si, swp_entries);
-		else
+		if (cluster) {
+			if (!(si->flags & SWP_FILE))
+				n_ret = swap_alloc_cluster(si, swp_entries);
+		} else
 			n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
 						    n_goal, swp_entries);
 		spin_unlock(&si->lock);