Re: [PATCH v2] mm, hotplug: move init_currently_empty_zone() under zone_span_lock protection

[David Hildenbrand <david@redhat.com>]

On 22.11.18 11:12, Wei Yang wrote:
> During online_pages phase, pgdat->nr_zones will be updated in case this
> zone is empty.
> 
> Currently the online_pages phase is protected by the global lock
> mem_hotplug_begin(), which ensures there is no contention during the
> update of nr_zones. But this global lock introduces scalability issues.
> 
> This patch is a preparation for removing the global lock during
> online_pages phase. Also this patch changes the documentation of
> node_size_lock to include the protectioin of nr_zones.

I looked into locking recently, and there is more to it.

Please read:

commit dee6da22efac451d361f5224a60be2796d847b51
Author: David Hildenbrand <david@redhat.com>
Date:   Tue Oct 30 15:10:44 2018 -0700

    memory-hotplug.rst: add some details about locking internals
    
    Let's document the magic a bit, especially why device_hotplug_lock is
    required when adding/removing memory and how it all play together with
    requests to online/offline memory from user space.

Short summary: Onlining/offlining of memory requires the device_hotplug_lock
as of now.

mem_hotplug_begin() is just an internal optimization. (we don't want
 everybody to take the device lock)



> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> ---
> v2:
>   * commit log changes
>   * modify the code in move_pfn_range_to_zone() instead of in
>     init_currently_empty_zone()
>   * documentation change
> 
> ---
>  include/linux/mmzone.h | 7 ++++---
>  mm/memory_hotplug.c    | 5 ++---
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 68d7b558924b..1bb749bee284 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -636,9 +636,10 @@ typedef struct pglist_data {
>  #endif
>  #if defined(CONFIG_MEMORY_HOTPLUG) || defined(CONFIG_DEFERRED_STRUCT_PAGE_INIT)
>  	/*
> -	 * Must be held any time you expect node_start_pfn, node_present_pages
> -	 * or node_spanned_pages stay constant.  Holding this will also
> -	 * guarantee that any pfn_valid() stays that way.
> +	 * Must be held any time you expect node_start_pfn,
> +	 * node_present_pages, node_spanned_pages or nr_zones stay constant.
> +	 * Holding this will also guarantee that any pfn_valid() stays that
> +	 * way.
>  	 *
>  	 * pgdat_resize_lock() and pgdat_resize_unlock() are provided to
>  	 * manipulate node_size_lock without checking for CONFIG_MEMORY_HOTPLUG
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 61972da38d93..f626e7e5f57b 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -742,14 +742,13 @@ void __ref move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
>  	int nid = pgdat->node_id;
>  	unsigned long flags;
>  
> -	if (zone_is_empty(zone))
> -		init_currently_empty_zone(zone, start_pfn, nr_pages);
> -
>  	clear_zone_contiguous(zone);
>  
>  	/* TODO Huh pgdat is irqsave while zone is not. It used to be like that before */
>  	pgdat_resize_lock(pgdat, &flags);
>  	zone_span_writelock(zone);
> +	if (zone_is_empty(zone))
> +		init_currently_empty_zone(zone, start_pfn, nr_pages);
>  	resize_zone_range(zone, start_pfn, nr_pages);
>  	zone_span_writeunlock(zone);
>  	resize_pgdat_range(pgdat, start_pfn, nr_pages);
> 


-- 

Thanks,

David / dhildenb