From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Subject: mm/memory_hotplug: remove broken locking of zone PCP structures during 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/20210412090346.GQ3697@xxxxxxxxxxxxxxxxxxx Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/page_alloc.c | 4 ---- 1 file changed, 4 deletions(-) --- a/mm/page_alloc.c~mm-memory_hotplug-make-unpopulated-zones-pcp-structures-unreachable-during-hot-remove +++ a/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 _
