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@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 81db389..b012805 100644 (file)
--- a/mm/page_alloc.c
+++ b/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