Subject: [PATCH v1 01/10] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes

From: David Hildenbrand <[email-masked]>
Date: Thu, 24 Oct 2019 14:09:29 +0200

Our onlining/offlining code is unnecessarily complicated. Only memory
blocks added during boot can have holes (a range that is not
IORESOURCE_SYSTEM_RAM). Hotplugged memory never has holes (e.g., see
add_memory_resource()). All boot memory is alread online.

Therefore, when we stop allowing to offline memory blocks with holes, we
implicitly no longer have to deal with onlining 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 (unmovable data might have easily ended up on that memory during
boot). So stopping to do that should not really hurt (+ people are not
even aware of a setup where that used to work and that the existing code
still works correctly with memory holes). For the use case of offlining
memory to unplug DIMMs, we should see no change. (holes on DIMMs would be
weird).

Please note that hardware errors (PG_hwpoison) are not memory holes and
not affected by this change when offlining.

Cc: Andrew Morton <[email-masked]>
Cc: Michal Hocko <[email-masked]>
Cc: Oscar Salvador <[email-masked]>
Cc: Pavel Tatashin <[email-masked]>
Cc: Dan Williams <[email-masked]>
Cc: Anshuman Khandual <[email-masked]>
Signed-off-by: David Hildenbrand <[email-masked]>
---
 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..8d81730cf036 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();
 
+	/*
+	 * Don't allow to offline memory blocks that contain holes.
+	 * Consecuently, memory blocks with holes can never get onlined
+	 * (hotplugged memory has no holes and all boot memory is online).
+	 * This allows to simplify the onlining/offlining code quite a lot.
+	 */
+	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