mm/memory_hotplug: remove broken locking of zone PCP structures durin…

…g hot remove

zone_pcp_reset allegedly protects against a race with drain_pages using
local_irq_save but this is bogus.  local_irq_save only operates on the
local CPU.  If memory hotplug is running on CPU A and drain_pages is
running on CPU B, disabling IRQs on CPU A does not affect CPU B and
offers no protection.

This patch deletes IRQ disable/enable on the grounds that IRQs protect
nothing and assumes the existing hotplug paths guarantees the PCP cannot
be used after zone_pcp_enable().  That should be the case already
because all the pages have been freed and there is no page to put on the
PCP lists.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_alloc.c

@@ -9020,12 +9020,9 @@ void zone_pcp_enable(struct zone *zone)
 
 void zone_pcp_reset(struct zone *zone)
 {
-	unsigned long flags;
 	int cpu;
 	struct per_cpu_pageset *pset;
 
-	/* avoid races with drain_pages()  */
-	local_irq_save(flags);
 	if (zone->pageset != &boot_pageset) {
 		for_each_online_cpu(cpu) {
 			pset = per_cpu_ptr(zone->pageset, cpu);
@@ -9034,7 +9031,6 @@ void zone_pcp_reset(struct zone *zone)
 		free_percpu(zone->pageset);
 		zone->pageset = &boot_pageset;
 	}
-	local_irq_restore(flags);
 }
 
 #ifdef CONFIG_MEMORY_HOTREMOVE