Re: [PATCH v1] drivers/base/node.c: Simplify unregister_memory_block_under_nodes()

[Michal Hocko <mhocko@kernel.org>]

On Thu 18-07-19 16:22:39, David Hildenbrand wrote:
> We don't allow to offline memory block devices that belong to multiple
> numa nodes. Therefore, such devices can never get removed. It is
> sufficient to process a single node when removing the memory block.
> 
> Remember for each memory block if it belongs to no, a single, or mixed
> nodes, so we can use that information to skip unregistering or print a
> warning (essentially a safety net to catch BUGs).

I do not really like NUMA_NO_NODE - 1 thing. This is yet another invalid
node that is magic. Why should we even care? In other words why is this
patch an improvement?

> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  drivers/base/memory.c  |  1 +
>  drivers/base/node.c    | 40 ++++++++++++++++------------------------
>  include/linux/memory.h |  4 +++-
>  3 files changed, 20 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 20c39d1bcef8..154d5d4a0779 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -674,6 +674,7 @@ static int init_memory_block(struct memory_block **memory,
>  	mem->state = state;
>  	start_pfn = section_nr_to_pfn(mem->start_section_nr);
>  	mem->phys_device = arch_get_memory_phys_device(start_pfn);
> +	mem->nid = NUMA_NO_NODE;
>  
>  	ret = register_memory(mem);
>  
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 75b7e6f6535b..29d27b8d5fda 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -759,8 +759,6 @@ static int register_mem_sect_under_node(struct memory_block *mem_blk,
>  	int ret, nid = *(int *)arg;
>  	unsigned long pfn, sect_start_pfn, sect_end_pfn;
>  
> -	mem_blk->nid = nid;
> -
>  	sect_start_pfn = section_nr_to_pfn(mem_blk->start_section_nr);
>  	sect_end_pfn = section_nr_to_pfn(mem_blk->end_section_nr);
>  	sect_end_pfn += PAGES_PER_SECTION - 1;
> @@ -789,6 +787,13 @@ static int register_mem_sect_under_node(struct memory_block *mem_blk,
>  			if (page_nid != nid)
>  				continue;
>  		}
> +
> +		/* this memory block spans this node */
> +		if (mem_blk->nid == NUMA_NO_NODE)
> +			mem_blk->nid = nid;
> +		else
> +			mem_blk->nid = NUMA_NO_NODE - 1;
> +
>  		ret = sysfs_create_link_nowarn(&node_devices[nid]->dev.kobj,
>  					&mem_blk->dev.kobj,
>  					kobject_name(&mem_blk->dev.kobj));
> @@ -804,32 +809,19 @@ static int register_mem_sect_under_node(struct memory_block *mem_blk,
>  }
>  
>  /*
> - * Unregister memory block device under all nodes that it spans.
> - * Has to be called with mem_sysfs_mutex held (due to unlinked_nodes).
> + * Unregister a memory block device under the node it spans. Memory blocks
> + * with multiple nodes cannot be offlined and therefore also never be removed.
>   */
>  void unregister_memory_block_under_nodes(struct memory_block *mem_blk)
>  {
> -	unsigned long pfn, sect_start_pfn, sect_end_pfn;
> -	static nodemask_t unlinked_nodes;
> -
> -	nodes_clear(unlinked_nodes);
> -	sect_start_pfn = section_nr_to_pfn(mem_blk->start_section_nr);
> -	sect_end_pfn = section_nr_to_pfn(mem_blk->end_section_nr);
> -	for (pfn = sect_start_pfn; pfn <= sect_end_pfn; pfn++) {
> -		int nid;
> +	if (mem_blk->nid == NUMA_NO_NODE ||
> +	    WARN_ON_ONCE(mem_blk->nid == NUMA_NO_NODE - 1))
> +		return;
>  
> -		nid = get_nid_for_pfn(pfn);
> -		if (nid < 0)
> -			continue;
> -		if (!node_online(nid))
> -			continue;
> -		if (node_test_and_set(nid, unlinked_nodes))
> -			continue;
> -		sysfs_remove_link(&node_devices[nid]->dev.kobj,
> -			 kobject_name(&mem_blk->dev.kobj));
> -		sysfs_remove_link(&mem_blk->dev.kobj,
> -			 kobject_name(&node_devices[nid]->dev.kobj));
> -	}
> +	sysfs_remove_link(&node_devices[mem_blk->nid]->dev.kobj,
> +		 kobject_name(&mem_blk->dev.kobj));
> +	sysfs_remove_link(&mem_blk->dev.kobj,
> +		 kobject_name(&node_devices[mem_blk->nid]->dev.kobj));
>  }
>  
>  int link_mem_sections(int nid, unsigned long start_pfn, unsigned long end_pfn)
> diff --git a/include/linux/memory.h b/include/linux/memory.h
> index 02e633f3ede0..c91af10d5fb4 100644
> --- a/include/linux/memory.h
> +++ b/include/linux/memory.h
> @@ -33,7 +33,9 @@ struct memory_block {
>  	void *hw;			/* optional pointer to fw/hw data */
>  	int (*phys_callback)(struct memory_block *);
>  	struct device dev;
> -	int nid;			/* NID for this memory block */
> +	int nid;			/* NID for this memory block.
> +					   - NUMA_NO_NODE: uninitialized
> +					   - NUMA_NO_NODE - 1: mixed nodes */
>  };
>  
>  int arch_get_memory_phys_device(unsigned long start_pfn);
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs