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* [PATCH RFC] mm,memory_hotplug: Drop unneeded locking
@ 2021-05-28  7:51 Oscar Salvador
  2021-05-28  8:50 ` David Hildenbrand
  0 siblings, 1 reply; 2+ messages in thread
From: Oscar Salvador @ 2021-05-28  7:51 UTC (permalink / raw)
  To: Andrew Morton
  Cc: David Hildenbrand, Michal Hocko, Anshuman Khandual,
	Vlastimil Babka, Pavel Tatashin, linux-mm, linux-kernel,
	Oscar Salvador

Currently, memory-hotplug code takes zone's span_writelock
and pgdat's resize_lock when resizing the node/zone's spanned
pages via {move_pfn_range_to_zone(),remove_pfn_range_from_zone()}
and when resizing node and zone's present pages via

These locks are also taken during the initialization of the system
at boot time, where it protects parallel struct page initialization,
but they should not really be needed in memory-hotplug where all
operations are a) synchronized on device level and b) serialized by
the mem_hotplug_lock lock.

Signed-off-by: Oscar Salvador <>
During the review of [1], it was brought up that we might need to revisit
the locking scheme in memory-hotplug code wrt. node and zone locks.
Given that memory-hotplug is serialized and synchronized by memory-hotplug lock
and the device, I do not think we need to hold zone/node's resize lock anymore.
After the system has been brought up, memory-hotplug is the only code allow to
touch {spanned,present}_pages, so it cannot really go off when serialized by its
own lock. Or am I missing something?
The only think I am not really sure is: if the from remove_pfn_range_from_zone,
shrink_zone_span, update_pgdat_span and move_pfn_range_to_zone can really go
(because those functions are already serialized by the memory-hotplug lock),
what about mhp_(de)init_memmap_on_memory that calls those functions?
Those are not protected by the lock. I have to confess that when it comes to locking
in this code, I always have to scratch my head.


 mm/memory_hotplug.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 075b34803fec..6edd7e267cad 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -329,7 +329,6 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
 	unsigned long pfn;
 	int nid = zone_to_nid(zone);
-	zone_span_writelock(zone);
 	if (zone->zone_start_pfn == start_pfn) {
 		 * If the section is smallest section in the zone, it need
@@ -362,7 +361,6 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
 			zone->spanned_pages = 0;
-	zone_span_writeunlock(zone);
 static void update_pgdat_span(struct pglist_data *pgdat)
@@ -424,10 +422,8 @@ void __ref remove_pfn_range_from_zone(struct zone *zone,
-	pgdat_resize_lock(zone->zone_pgdat, &flags);
 	shrink_zone_span(zone, start_pfn, start_pfn + nr_pages);
-	pgdat_resize_unlock(zone->zone_pgdat, &flags);
@@ -639,14 +635,10 @@ void __ref move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
 	/* TODO Huh pgdat is irqsave while zone is not. It used to be like that before */
-	pgdat_resize_lock(pgdat, &flags);
-	zone_span_writelock(zone);
 	if (zone_is_empty(zone))
 		init_currently_empty_zone(zone, start_pfn, nr_pages);
 	resize_zone_range(zone, start_pfn, nr_pages);
-	zone_span_writeunlock(zone);
 	resize_pgdat_range(pgdat, start_pfn, nr_pages);
-	pgdat_resize_unlock(pgdat, &flags);
 	 * Subsection population requires care in pfn_to_online_page().
@@ -739,9 +731,7 @@ void adjust_present_page_count(struct zone *zone, long nr_pages)
 	unsigned long flags;
 	zone->present_pages += nr_pages;
-	pgdat_resize_lock(zone->zone_pgdat, &flags);
 	zone->zone_pgdat->node_present_pages += nr_pages;
-	pgdat_resize_unlock(zone->zone_pgdat, &flags);
 int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,

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2021-05-28  7:51 [PATCH RFC] mm,memory_hotplug: Drop unneeded locking Oscar Salvador
2021-05-28  8:50 ` David Hildenbrand

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