mm/page_isolation: drop WARN_ON_ONCE() in set_migratetype_isolate()

Inside has_unmovable_pages(), we have a comment describing how unmovable
data could end up in ZONE_MOVABLE - via "movablecore".  Also, besides
checking if the first page in the pageblock is reserved, we don't perform
any further checks in case of ZONE_MOVABLE.

In case of memory offlining, we set REPORT_FAILURE, properly dump_page()
the page and handle the error gracefully.  alloc_contig_pages() users
currently never allocate from ZONE_MOVABLE.  E.g., hugetlb uses
alloc_contig_pages() for the allocation of gigantic pages only, which will
never end up on the MOVABLE zone (see htlb_alloc_mask()).
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Qian Cai <cai@lca.pw>
Link: http://lkml.kernel.org/r/20200816125333.7434-4-david@redhat.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_isolation.c

@@ -57,10 +57,7 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
 	spin_unlock_irqrestore(&zone->lock, flags);
 	if (!ret) {
 		drain_all_pages(zone);
-	} else {
-		WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
-
-		if ((isol_flags & REPORT_FAILURE) && unmovable)
+	} else if ((isol_flags & REPORT_FAILURE) && unmovable) {
 		/*
 		 * printk() with zone->lock held will likely trigger a
 		 * lockdep splat, so defer it here.