dm kcopyd: use __GFP_HIGHMEM when allocating pages

dm-kcopyd doesn't access the allocated pages directly, it only passes
them to dm-io which adds them to a bio list - thus, we can allocate
the pages from high memory. This will reduce pressure on the low
memory when there are a large number of kcopyd jobs in progress.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-kcopyd.c

@@ -219,7 +219,7 @@ static struct page_list *alloc_pl(gfp_t gfp)
 	if (!pl)
 		return NULL;
 
-	pl->page = alloc_page(gfp);
+	pl->page = alloc_page(gfp | __GFP_HIGHMEM);
 	if (!pl->page) {
 		kfree(pl);
 		return NULL;