Re: [PATCH] mm/migrate: rework hotplug_memory_notifier() stub

[Michal Hocko <mhocko@suse.com>]

On Wed 13-10-21 16:39:38, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The previous change added a compile time warning for the
> CONFIG_MEMORY_HOTPLUG=n case:
> 
> mm/migrate.c:3216:22: error: 'migrate_on_reclaim_callback' defined but not used [-Werror=unused-function]
>  3216 | static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
>       |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/migrate.c:3197:13: error: 'set_migration_target_nodes' defined but not used [-Werror=unused-function]
>  3197 | static void set_migration_target_nodes(void)
>       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Rework the hotplug_memory_notifier() stub implementation into
> an inline function so it always references its argument but
> ignores it, leading the compiler to silently drop the hotplug
> memory notifier function and further code referenced from it.
> 
> To do this, the #ifdef around those functions need to be removed
> as well, but this tends to help with readability and build coverage.
> 
> Fixes: dd3e24b84bc1 ("mm/migrate: add CPU hotplug to demotion #ifdef")

This is not a sha1 in the Linus tree. I suspect this comes from
linux-next and akpm patch queue. Those sha1s are not stable so it is
better to not reference them.

Can we fold that into patch directly?

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

The fix looks good to me.
Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
>  include/linux/memory.h | 5 ++++-
>  mm/migrate.c           | 6 +-----
>  mm/page_ext.c          | 4 +---
>  mm/slab.c              | 4 ++--
>  4 files changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/memory.h b/include/linux/memory.h
> index a216829df280..95af54f585a1 100644
> --- a/include/linux/memory.h
> +++ b/include/linux/memory.h
> @@ -125,7 +125,10 @@ static inline int memory_notify(unsigned long val, void *v)
>  {
>  	return 0;
>  }
> -#define hotplug_memory_notifier(fn, pri)	({ 0; })
> +static inline int hotplug_memory_notifier(notifier_fn_t fn, int pri)
> +{
> +	return 0;
> +}
>  /* These aren't inline functions due to a GCC bug. */
>  #define register_hotmemory_notifier(nb)    ({ (void)(nb); 0; })
>  #define unregister_hotmemory_notifier(nb)  ({ (void)(nb); })
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 7769abac8aad..32303d517e10 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -3059,7 +3059,7 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
>  EXPORT_SYMBOL(migrate_vma_finalize);
>  #endif /* CONFIG_DEVICE_PRIVATE */
>  
> -#if defined(CONFIG_MEMORY_HOTPLUG) || defined(CONFIG_HOTPLUG_CPU)
> +#if defined(CONFIG_HOTPLUG_CPU)
>  /* Disable reclaim-based migration. */
>  static void __disable_all_migrate_targets(void)
>  {
> @@ -3201,7 +3201,6 @@ static void set_migration_target_nodes(void)
>  	put_online_mems();
>  }
>  
> -#if defined(CONFIG_MEMORY_HOTPLUG)
>  /*
>   * This leaves migrate-on-reclaim transiently disabled between
>   * the MEM_GOING_OFFLINE and MEM_OFFLINE events.  This runs
> @@ -3258,9 +3257,7 @@ static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
>  
>  	return notifier_from_errno(0);
>  }
> -#endif /* CONFIG_MEMORY_HOTPLUG */
>  
> -#ifdef CONFIG_HOTPLUG_CPU
>  /*
>   * React to hotplug events that might affect the migration targets
>   * like events that online or offline NUMA nodes.
> @@ -3302,4 +3299,3 @@ static int __init migrate_on_reclaim_init(void)
>  }
>  late_initcall(migrate_on_reclaim_init);
>  #endif /* CONFIG_HOTPLUG_CPU */
> -#endif /* CONFIG_MEMORY_HOTPLUG || CONFIG_HOTPLUG_CPU */
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 106079e67c2e..6242afb24d84 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -269,7 +269,7 @@ static int __meminit init_section_page_ext(unsigned long pfn, int nid)
>  	total_usage += table_size;
>  	return 0;
>  }
> -#ifdef CONFIG_MEMORY_HOTPLUG
> +
>  static void free_page_ext(void *addr)
>  {
>  	if (is_vmalloc_addr(addr)) {
> @@ -374,8 +374,6 @@ static int __meminit page_ext_callback(struct notifier_block *self,
>  	return notifier_from_errno(ret);
>  }
>  
> -#endif
> -
>  void __init page_ext_init(void)
>  {
>  	unsigned long pfn;
> diff --git a/mm/slab.c b/mm/slab.c
> index 01775fe0e002..da132a9ae6f8 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1095,7 +1095,7 @@ static int slab_offline_cpu(unsigned int cpu)
>  	return 0;
>  }
>  
> -#if defined(CONFIG_NUMA) && defined(CONFIG_MEMORY_HOTPLUG)
> +#if defined(CONFIG_NUMA)
>  /*
>   * Drains freelist for a node on each slab cache, used for memory hot-remove.
>   * Returns -EBUSY if all objects cannot be drained so that the node is not
> @@ -1157,7 +1157,7 @@ static int __meminit slab_memory_callback(struct notifier_block *self,
>  out:
>  	return notifier_from_errno(ret);
>  }
> -#endif /* CONFIG_NUMA && CONFIG_MEMORY_HOTPLUG */
> +#endif /* CONFIG_NUMA */
>  
>  /*
>   * swap the static kmem_cache_node with kmalloced memory
> -- 
> 2.29.2

-- 
Michal Hocko
SUSE Labs