mm: vmscan: remove remains of kswapd-managed zone->all_unreclaimable

shrink_zones() has a special branch to skip the all_unreclaimable()
check during hibernation, because a frozen kswapd can't mark a zone
unreclaimable.

But ever since commit 6e543d57 ("mm: vmscan: fix
do_try_to_free_pages() livelock"), determining a zone to be
unreclaimable is done by directly looking at its scan history and no
longer relies on kswapd setting the per-zone flag.

Remove this branch and let shrink_zones() check the reclaimability of
the target zones regardless of hibernation state.
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: KOSAKI Motohiro <Kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2534,14 +2534,6 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 	if (sc->nr_reclaimed)
 		return sc->nr_reclaimed;
 
-	/*
-	 * As hibernation is going on, kswapd is freezed so that it can't mark
-	 * the zone into all_unreclaimable. Thus bypassing all_unreclaimable
-	 * check.
-	 */
-	if (oom_killer_disabled)
-		return 0;
-
 	/* Aborted reclaim to try compaction? don't OOM, then */
 	if (aborted_reclaim)
 		return 1;