mm/memory_hotplug: simplify page onlining

We don't allow to offline memory with holes, all boot memory is online,
and all hotplugged memory cannot have holes.

We can now simplify onlining of pages.  As we only allow to online/offline
full sections and sections always span full MAX_ORDER_NR_PAGES, we can
just process MAX_ORDER - 1 pages without further special handling.

The number of onlined pages simply corresponds to the number of pages we
were requested to online.

While at it, refine the comment regarding the callback not exposing all
pages to the buddy.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Charan Teja Reddy <charante@codeaurora.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michel Lespinasse <walken@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200819175957.28465-8-david@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: davidhildenbrand

mm/memory_hotplug.c

@@ -625,31 +625,22 @@ void generic_online_page(struct page *page, unsigned int order)
 }
 EXPORT_SYMBOL_GPL(generic_online_page);
 
-static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
-			void *arg)
+static void online_pages_range(unsigned long start_pfn, unsigned long nr_pages)
 {
 	const unsigned long end_pfn = start_pfn + nr_pages;
 	unsigned long pfn;
-	int order;
 
 	/*
-	 * Online the pages. The callback might decide to keep some pages
-	 * PG_reserved (to add them to the buddy later), but we still account
-	 * them as being online/belonging to this zone ("present").
+	 * Online the pages in MAX_ORDER - 1 aligned chunks. The callback might
+	 * decide to not expose all pages to the buddy (e.g., expose them
+	 * later). We account all pages as being online and belonging to this
+	 * zone ("present").
 	 */
-	for (pfn = start_pfn; pfn < end_pfn; pfn += 1ul << order) {
-		order = min(MAX_ORDER - 1, get_order(PFN_PHYS(end_pfn - pfn)));
-		/* __free_pages_core() wants pfns to be aligned to the order */
-		if (WARN_ON_ONCE(!IS_ALIGNED(pfn, 1ul << order)))
-			order = 0;
-		(*online_page_callback)(pfn_to_page(pfn), order);
-	}
+	for (pfn = start_pfn; pfn < end_pfn; pfn += MAX_ORDER_NR_PAGES)
+		(*online_page_callback)(pfn_to_page(pfn), MAX_ORDER - 1);
 
 	/* mark all involved sections as online */
 	online_mem_sections(start_pfn, end_pfn);
-
-	*(unsigned long *)arg += nr_pages;
-	return 0;
 }
 
 /* check which state of node_states will be changed when online memory */
@@ -803,7 +794,6 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
 		       int online_type, int nid)
 {
 	unsigned long flags;
-	unsigned long onlined_pages = 0;
 	struct zone *zone;
 	int need_zonelists_rebuild = 0;
 	int ret;
@@ -839,19 +829,11 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
 		setup_zone_pageset(zone);
 	}
 
-	ret = walk_system_ram_range(pfn, nr_pages, &onlined_pages,
-		online_pages_range);
-	if (ret) {
-		/* not a single memory resource was applicable */
-		if (need_zonelists_rebuild)
-			zone_pcp_reset(zone);
-		goto failed_addition;
-	}
-
-	zone->present_pages += onlined_pages;
+	online_pages_range(pfn, nr_pages);
+	zone->present_pages += nr_pages;
 
 	pgdat_resize_lock(zone->zone_pgdat, &flags);
-	zone->zone_pgdat->node_present_pages += onlined_pages;
+	zone->zone_pgdat->node_present_pages += nr_pages;
 	pgdat_resize_unlock(zone->zone_pgdat, &flags);
 
 	/*