linux-kernel - Re: [PATCH RFC 1/2] mm/memory_hotplug: no need to init new pgdat with node_start

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Date:   Tue, 21 Apr 2020 14:30:11 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>
Subject: Re: [PATCH RFC 1/2] mm/memory_hotplug: no need to init new pgdat
 with node_start_pfn

Sorry for the late reply

On Thu 16-04-20 12:47:06, David Hildenbrand wrote:
> A hotadded node/pgdat will span no pages at all, until memory is moved to
> the zone/node via move_pfn_range_to_zone() -> resize_pgdat_range - e.g.,
> when onlining memory blocks. We don't have to initialize the
> node_start_pfn to the memory we are adding.

You are right that the node is empty at this phase but that is already
reflected by zero present pages (hmm, I do not see spanned pages to be
set 0 though). What I am missing here is why this is an improvement. The
new node is already visible here and I do not see why we hide the
information we already know.
 
> Note: we'll also end up with pgdat->node_start_pfn == 0 when offlined the
> last memory block belonging to a node (via remove_pfn_range_from_zone()->
> update_pgdat_span()).
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Baoquan He <bhe@...hat.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/memory_hotplug.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 47cf6036eb31..9b15ce465be2 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -866,10 +866,9 @@ static void reset_node_present_pages(pg_data_t *pgdat)
>  }
>  
>  /* we are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG */
> -static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
> +static pg_data_t __ref *hotadd_new_pgdat(int nid)
>  {
>  	struct pglist_data *pgdat;
> -	unsigned long start_pfn = PFN_DOWN(start);
>  
>  	pgdat = NODE_DATA(nid);
>  	if (!pgdat) {
> @@ -899,9 +898,8 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>  	}
>  
>  	/* we can use NODE_DATA(nid) from here */
> -
>  	pgdat->node_id = nid;
> -	pgdat->node_start_pfn = start_pfn;
> +	pgdat->node_start_pfn = 0;
>  
>  	/* init node's zones as empty zones, we don't have any present pages.*/
>  	free_area_init_core_hotplug(nid);
> @@ -936,7 +934,6 @@ static void rollback_node_hotadd(int nid)
>  /**
>   * try_online_node - online a node if offlined
>   * @nid: the node ID
> - * @start: start addr of the node
>   * @set_node_online: Whether we want to online the node
>   * called by cpu_up() to online a node without onlined memory.
>   *
> @@ -945,7 +942,7 @@ static void rollback_node_hotadd(int nid)
>   * 0 -> the node is already online
>   * -ENOMEM -> the node could not be allocated
>   */
> -static int __try_online_node(int nid, u64 start, bool set_node_online)
> +static int __try_online_node(int nid, bool set_node_online)
>  {
>  	pg_data_t *pgdat;
>  	int ret = 1;
> @@ -953,7 +950,7 @@ static int __try_online_node(int nid, u64 start, bool set_node_online)
>  	if (node_online(nid))
>  		return 0;
>  
> -	pgdat = hotadd_new_pgdat(nid, start);
> +	pgdat = hotadd_new_pgdat(nid);
>  	if (!pgdat) {
>  		pr_err("Cannot online node %d due to NULL pgdat\n", nid);
>  		ret = -ENOMEM;
> @@ -977,7 +974,7 @@ int try_online_node(int nid)
>  	int ret;
>  
>  	mem_hotplug_begin();
> -	ret =  __try_online_node(nid, 0, true);
> +	ret =  __try_online_node(nid, true);
>  	mem_hotplug_done();
>  	return ret;
>  }
> @@ -1031,7 +1028,7 @@ int __ref add_memory_resource(int nid, struct resource *res)
>  	 */
>  	memblock_add_node(start, size, nid);
>  
> -	ret = __try_online_node(nid, start, false);
> +	ret = __try_online_node(nid, false);
>  	if (ret < 0)
>  		goto error;
>  	new_node = ret;
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs