Re: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove

[Vlastimil Babka]

On 4/12/21 2:08 PM, 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.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Yeah the irq disabling here is clearly bogus, so:

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

But I think Michal has a point that we might best leave the pagesets around, by
a future change. I'm have some doubts that even with your reordering of the
reset/destroy after zonelist rebuild in v1 they cant't be reachable. We have no
protection between zonelist rebuild and zonelist traversal, and that's why we
just leave pgdats around.

So I can imagine a task racing with memory hotremove might see watermarks as ok
in get_page_from_freelist() for the zone and proceeds to try_this_zone:, then
gets stalled/scheduled out while hotremove rebuilds the zonelist and destroys
the pcplists, then the first task is resumed and proceeds with rmqueue_pcplist().

So that's very rare thus not urgent, and this patch doesn't make it less rare so
not a reason to block it.

> ---
> 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
>