mm: compaction: don't require high order pages below min wmark

The min wmark should be satisfied with just 1 hugepage. And the other
wmarks should be adjusted accordingly. We need to succeed the low
wmark check if there's some significant amount of 0 order pages, but
we don't need plenty of high order pages because the PF_MEMALLOC paths
don't require those. Creating a ton of high order pages that cannot be
allocated by the high order allocation paths (no PF_MEMALLOC) is quite
wasteful because they can be splitted in lower order pages before
anybody has a chance to allocate them.

mm/page_alloc.c

@@ -1874,6 +1874,23 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
 
 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
 		return false;
+	if (!order)
+		return true;
+
+	/*
+	 * Don't require any high order page under the min
+	 * wmark. Invoking compaction to create lots of high order
+	 * pages below the min wmark is wasteful because those
+	 * hugepages cannot be allocated without PF_MEMALLOC and the
+	 * PF_MEMALLOC paths must not depend on high order allocations
+	 * to succeed.
+	 */
+	min = mark - z->watermark[WMARK_MIN];
+	WARN_ON(min < 0);
+	if (alloc_flags & ALLOC_HIGH)
+		min -= min / 2;
+	if (alloc_flags & ALLOC_HARDER)
+		min -= min / 4;
 	for (o = 0; o < order; o++) {
 		/* At the next order, this order's pages become unavailable */
 		free_pages -= z->free_area[o].nr_free << o;