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@xxxxxxxxxx>
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@xxxxxxxxx>
> ---
> 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
