On Mon 12-04-21 13:08:42, Mel Gorman wrote:
> 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.
Yes, that is the case since ec6e8c7e0314 ("mm, page_alloc: disable
pcplists during memory offline"). Prior to this commit the behavior was
undefined but full zone/node hotremove is rare enough that an existing
race was likely never observed.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
> Resending for email address correction and adding lists
>
> Changelog since v1
> o Minimal fix
>
> mm/page_alloc.c | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5e8aedb64b57..9bf0db982f14 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8952,12 +8952,9 @@ 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);
> @@ -8966,7 +8963,6 @@ void zone_pcp_reset(struct zone *zone)
> free_percpu(zone->pageset);
> zone->pageset = &boot_pageset;
> }
> - local_irq_restore(flags);
> }
>
> #ifdef CONFIG_MEMORY_HOTREMOVE
--
Michal Hocko
SUSE Labs
