vmscan: handle may_swap more strictly

vmscan: handle may_swap more strictly

Commit 2e2e425989080cc534fc0fca154cae515f971cf5 ("vmscan,memcg:
reintroduce sc->may_swap) add may_swap flag and handle it at
get_scan_ratio().

But the result of get_scan_ratio() is ignored when priority == 0, so anon
lru is scanned even if may_swap == 0 or nr_swap_pages == 0.  IMHO, this is
not an expected behavior.

As for memcg especially, because of this behavior many and many pages are
swapped-out just in vain when oom is invoked by mem+swap limit.

This patch is for handling may_swap flag more strictly.

Signed-off-by: Daisuke Nishimura <[email protected]>
Reviewed-by: KOSAKI Motohiro <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Balbir Singh <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>