Re: [PATCH 8/9] mm, page_alloc: drain all pcplists during memory offline

[David Hildenbrand]

On 22.09.20 16:37, Vlastimil Babka wrote:
> drain_all_pages() is optimized to only execute on cpus where pcplists are not
> empty. The check can however race with a free to pcplist that has not yet
> increased the pcp->count from 0 to 1. Make the drain optionally skip the racy
> check and drain on all cpus, and use it in memory offline context, where we
> want to make sure no isolated pages are left behind on pcplists.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  include/linux/gfp.h |  1 +
>  mm/memory_hotplug.c |  4 ++--
>  mm/page_alloc.c     | 29 ++++++++++++++++++++---------
>  3 files changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 67a0774e080b..cc52c5cc9fab 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -592,6 +592,7 @@ extern void page_frag_free(void *addr);
>  
>  void page_alloc_init(void);
>  void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp);
> +void __drain_all_pages(struct zone *zone, bool page_isolation);
>  void drain_all_pages(struct zone *zone);
>  void drain_local_pages(struct zone *zone);
>  
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 08f729922e18..bbde415b558b 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1524,7 +1524,7 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  		goto failed_removal;
>  	}
>  
> -	drain_all_pages(zone);
> +	__drain_all_pages(zone, true);
>  
>  	arg.start_pfn = start_pfn;
>  	arg.nr_pages = nr_pages;
> @@ -1588,7 +1588,7 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  		 */
>  		ret = test_pages_isolated(start_pfn, end_pfn, MEMORY_OFFLINE);
>  		if (ret)
> -			drain_all_pages(zone);
> +			__drain_all_pages(zone, true);
>  	} while (ret);
>  
>  	/* Mark all sections offline and remove free pages from the buddy. */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4e37bc3f6077..33cc35d152b1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2960,14 +2960,7 @@ static void drain_local_pages_wq(struct work_struct *work)
>  	preempt_enable();
>  }
>  
> -/*
> - * Spill all the per-cpu pages from all CPUs back into the buddy allocator.
> - *
> - * When zone parameter is non-NULL, spill just the single zone's pages.
> - *
> - * Note that this can be extremely slow as the draining happens in a workqueue.
> - */
> -void drain_all_pages(struct zone *zone)
> +void __drain_all_pages(struct zone *zone, bool force_all_cpus)
>  {
>  	int cpu;
>  
> @@ -3006,7 +2999,13 @@ void drain_all_pages(struct zone *zone)
>  		struct zone *z;
>  		bool has_pcps = false;
>  
> -		if (zone) {
> +		if (force_all_cpus) {
> +			/*
> +			 * The pcp.count check is racy, some callers need a
> +			 * guarantee that no cpu is missed.
> +			 */
> +			has_pcps = true;
> +		} else if (zone) {
>  			pcp = per_cpu_ptr(zone->pageset, cpu);
>  			if (pcp->pcp.count)
>  				has_pcps = true;
> @@ -3039,6 +3038,18 @@ void drain_all_pages(struct zone *zone)
>  	mutex_unlock(&pcpu_drain_mutex);
>  }
>  
> +/*
> + * Spill all the per-cpu pages from all CPUs back into the buddy allocator.
> + *
> + * When zone parameter is non-NULL, spill just the single zone's pages.
> + *
> + * Note that this can be extremely slow as the draining happens in a workqueue.
> + */
> +void drain_all_pages(struct zone *zone)
> +{
> +	__drain_all_pages(zone, false);
> +}
> +
>  #ifdef CONFIG_HIBERNATION
>  
>  /*
> 

Interesting race. Instead of this ugly __drain_all_pages() with a
boolean parameter, can we have two properly named functions to be used
in !page_alloc.c code without scratching your head what the difference is?

(yeah, coming up with a proper name is difficult. the one gives more
guarantees than the other, that cannot really be deducted from
"force_all_cpus" - maybe we can encode the actual semantics in the name)


-- 
Thanks,

David / dhildenb