[PATCH RFC v1 01/12] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes

[David Hildenbrand]

Our onlining/offlining code is unnecessarily complicated. Only memory
blocks added during boot can have holes. Hotplugged memory never has
holes. That memory is already online.

When we stop allowing to offline memory blocks with holes, we implicitly
stop to online memory blocks with holes.

This allows to simplify the code. For example, we no longer have to
worry about marking pages that fall into memory holes PG_reserved when
onlining memory. We can stop setting pages PG_reserved.

Offlining memory blocks added during boot is usually not guranteed to work
either way. So stopping to do that (if anybody really used and tested
this over the years) should not really hurt. For the use case of
offlining memory to unplug DIMMs, we should see no change. (holes on
DIMMs would be weird)

Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Oscar Salvador <osalvador at suse.de>
Cc: Pavel Tatashin <pasha.tatashin at soleen.com>
Cc: Dan Williams <dan.j.williams at intel.com>
Signed-off-by: David Hildenbrand <david at redhat.com>
---
 mm/memory_hotplug.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 561371ead39a..7210f4375279 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1447,10 +1447,19 @@ static void node_states_clear_node(int node, struct memory_notify *arg)
 		node_clear_state(node, N_MEMORY);
 }
 
+static int count_system_ram_pages_cb(unsigned long start_pfn,
+				     unsigned long nr_pages, void *data)
+{
+	unsigned long *nr_system_ram_pages = data;
+
+	*nr_system_ram_pages += nr_pages;
+	return 0;
+}
+
 static int __ref __offline_pages(unsigned long start_pfn,
 		  unsigned long end_pfn)
 {
-	unsigned long pfn, nr_pages;
+	unsigned long pfn, nr_pages = 0;
 	unsigned long offlined_pages = 0;
 	int ret, node, nr_isolate_pageblock;
 	unsigned long flags;
@@ -1461,6 +1470,20 @@ static int __ref __offline_pages(unsigned long start_pfn,
 
 	mem_hotplug_begin();
 
+	/*
+	 * We don't allow to offline memory blocks that contain holes
+	 * and consecuently don't allow to online memory blocks that contain
+	 * holes. This allows to simplify the code quite a lot and we don't
+	 * have to mess with PG_reserved pages for memory holes.
+	 */
+	walk_system_ram_range(start_pfn, end_pfn - start_pfn, &nr_pages,
+			      count_system_ram_pages_cb);
+	if (nr_pages != end_pfn - start_pfn) {
+		ret = -EINVAL;
+		reason = "memory holes";
+		goto failed_removal;
+	}
+
 	/* This makes hotplug much easier...and readable.
 	   we assume this for now. .*/
 	if (!test_pages_in_a_zone(start_pfn, end_pfn, &valid_start,
@@ -1472,7 +1495,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
 
 	zone = page_zone(pfn_to_page(valid_start));
 	node = zone_to_nid(zone);
-	nr_pages = end_pfn - start_pfn;
 
 	/* set above range as isolated */
 	ret = start_isolate_page_range(start_pfn, end_pfn,
-- 
2.21.0