Re: [PATCH 4/5] mm/memory_hotplug: Simplify node_states_check_changes_online

[Pasha Tatashin <Pavel.Tatashin@microsoft.com>]

On 9/19/18 6:08 AM, Oscar Salvador wrote:
> From: Oscar Salvador <osalvador@suse.de>
> 
> While looking at node_states_check_changes_online, I stumbled
> upon some confusing things.
> 
> Right after entering the function, we find this:
> 
> if (N_MEMORY == N_NORMAL_MEMORY)
>         zone_last = ZONE_MOVABLE;
> 
> This is wrong.
> N_MEMORY cannot really be equal to N_NORMAL_MEMORY.
> My guess is that this wanted to be something like:
> 
> if (N_NORMAL_MEMORY == N_HIGH_MEMORY)
> 
> to check if we have CONFIG_HIGHMEM.
> 
> Later on, in the CONFIG_HIGHMEM block, we have:
> 
> if (N_MEMORY == N_HIGH_MEMORY)
>         zone_last = ZONE_MOVABLE;
> 
> Again, this is wrong, and will never be evaluated to true.
> 
> Besides removing these wrong if statements, I simplified
> the function a bit.
> 
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
>  mm/memory_hotplug.c | 71 +++++++++++++++++------------------------------------
>  1 file changed, 23 insertions(+), 48 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 131c08106d54..ab3c1de18c5d 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -687,61 +687,36 @@ static void node_states_check_changes_online(unsigned long nr_pages,
>  	struct zone *zone, struct memory_notify *arg)
>  {
>  	int nid = zone_to_nid(zone);
> -	enum zone_type zone_last = ZONE_NORMAL;
>  
>  	/*
> -	 * If we have HIGHMEM or movable node, node_states[N_NORMAL_MEMORY]
> -	 * contains nodes which have zones of 0...ZONE_NORMAL,
> -	 * set zone_last to ZONE_NORMAL.
> -	 *
> -	 * If we don't have HIGHMEM nor movable node,
> -	 * node_states[N_NORMAL_MEMORY] contains nodes which have zones of
> -	 * 0...ZONE_MOVABLE, set zone_last to ZONE_MOVABLE.
> +	 * zone_for_pfn_range() can only return a zone within
> +	 * (0..ZONE_NORMAL] or ZONE_MOVABLE.

But what if that changes, will this function need to change as well?

> +	 * If the zone is within the range (0..ZONE_NORMAL],
> +	 * we need to check if:
> +	 * 1) We need to set the node for N_NORMAL_MEMORY
> +	 * 2) On CONFIG_HIGHMEM systems, we need to also set
> +	 *    the node for N_HIGH_MEMORY.
> +	 * 3) On !CONFIG_HIGHMEM, we can disregard N_HIGH_MEMORY,
> +	 *    as N_HIGH_MEMORY falls back to N_NORMAL_MEMORY.
>  	 */
> -	if (N_MEMORY == N_NORMAL_MEMORY)
> -		zone_last = ZONE_MOVABLE;
>  
> -	/*
> -	 * if the memory to be online is in a zone of 0...zone_last, and
> -	 * the zones of 0...zone_last don't have memory before online, we will
> -	 * need to set the node to node_states[N_NORMAL_MEMORY] after
> -	 * the memory is online.
> -	 */
> -	if (zone_idx(zone) <= zone_last && !node_state(nid, N_NORMAL_MEMORY))
> -		arg->status_change_nid_normal = nid;
> -	else
> -		arg->status_change_nid_normal = -1;
> -
> -#ifdef CONFIG_HIGHMEM
> -	/*
> -	 * If we have movable node, node_states[N_HIGH_MEMORY]
> -	 * contains nodes which have zones of 0...ZONE_HIGHMEM,
> -	 * set zone_last to ZONE_HIGHMEM.
> -	 *
> -	 * If we don't have movable node, node_states[N_NORMAL_MEMORY]
> -	 * contains nodes which have zones of 0...ZONE_MOVABLE,
> -	 * set zone_last to ZONE_MOVABLE.
> -	 */
> -	zone_last = ZONE_HIGHMEM;
> -	if (N_MEMORY == N_HIGH_MEMORY)
> -		zone_last = ZONE_MOVABLE;
> +	if (zone_idx(zone) <= ZONE_NORMAL) {
> +		if (!node_state(nid, N_NORMAL_MEMORY))
> +			arg->status_change_nid_normal = nid;
> +		else
> +			arg->status_change_nid_normal = -1;
>  
> -	if (zone_idx(zone) <= zone_last && !node_state(nid, N_HIGH_MEMORY))
> -		arg->status_change_nid_high = nid;
> -	else
> -		arg->status_change_nid_high = -1;
> -#else
> -	/*
> -	 * When !CONFIG_HIGHMEM, N_HIGH_MEMORY equals N_NORMAL_MEMORY
> -	 * so setting the node for N_NORMAL_MEMORY is enough.
> -	 */
> -	arg->status_change_nid_high = -1;
> -#endif
> +		if (IS_ENABLED(CONFIG_HIGHMEM)) {
> +			if (!node_state(nid, N_HIGH_MEMORY))
> +				arg->status_change_nid_high = nid;

Should not we have:
			else
				arg->status_change_nid_high = -1; ?

> +		} else
> +			arg->status_change_nid_high = -1;


I prefer to have braces in else part as well when if has braces.

> +	}
>  
>  	/*
> -	 * if the node don't have memory befor online, we will need to
> -	 * set the node to node_states[N_MEMORY] after the memory
> -	 * is online.
> +	 * if the node doesn't have memory before onlining it, we will need
> +	 * to set the node to node_states[N_MEMORY] after the memory
> +	 * gets onlined.
>  	 */
>  	if (!node_state(nid, N_MEMORY))
>  		arg->status_change_nid = nid;
> 

I think it is simpler to have something like this:

        int nid = zone_to_nid(zone);

        arg->status_change_nid_high = -1;
        arg->status_change_nid = -1;
        arg->status_change_nid = -1;

        if (!node_state(nid, N_MEMORY))
                arg->status_change_nid = nid; 
        if (zone_idx(zone) <= ZONE_NORMAL && !node_state(nid, N_NORMAL_MEMORY))
                arg->status_change_nid_normal = nid; 
#ifdef CONFIG_HIGHMEM
        if (zone_idx(zone) <= N_HIGH_MEMORY && !node_state(nid, N_HIGH_MEMORY))
                arg->status_change_nid_high = nid; 
#endif


Pavel