linux-kernel - Re: [PATCH 5/5] mm/memory_hotplug: Clean up node_states_check_changes

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Date:   Fri, 21 Sep 2018 00:38:59 +0000
From:   Pasha Tatashin <Pavel.Tatashin@...rosoft.com>
To:     Oscar Salvador <osalvador@...hadventures.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "mhocko@...e.com" <mhocko@...e.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
        "yasu.isimatu@...il.com" <yasu.isimatu@...il.com>,
        "malat@...ian.org" <malat@...ian.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH 5/5] mm/memory_hotplug: Clean up
 node_states_check_changes_offline

On 9/19/18 6:08 AM, Oscar Salvador wrote:
> From: Oscar Salvador <osalvador@...e.de>
> 
> This patch, as the previous one, gets rid of the wrong if statements.
> While at it, I realized that the comments are sometimes very confusing,
> to say the least, and wrong.
> For example:
> 
> ---
> zone_last = ZONE_MOVABLE;
> 
> /*
>  * check whether node_states[N_HIGH_MEMORY] will be changed
>  * If we try to offline the last present @nr_pages from the node,
>  * we can determind we will need to clear the node from
>  * node_states[N_HIGH_MEMORY].
>  */
> 
> for (; zt <= zone_last; zt++)
> 	present_pages += pgdat->node_zones[zt].present_pages;
> if (nr_pages >= present_pages)
> 	arg->status_change_nid = zone_to_nid(zone);
> else
> 	arg->status_change_nid = -1;
> ---
> 
> In case the node gets empry, it must be removed from N_MEMORY.
> We already check N_HIGH_MEMORY a bit above within the CONFIG_HIGHMEM
> ifdef code.
> Not to say that status_change_nid is for N_MEMORY, and not for
> N_HIGH_MEMORY.
> 
> So I re-wrote some of the comments to what I think is better.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>  mm/memory_hotplug.c | 71 +++++++++++++++++++++--------------------------------
>  1 file changed, 28 insertions(+), 43 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index ab3c1de18c5d..15ecf3d7a554 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1485,51 +1485,36 @@ static void node_states_check_changes_offline(unsigned long nr_pages,
>  {
>  	struct pglist_data *pgdat = zone->zone_pgdat;
>  	unsigned long present_pages = 0;
> -	enum zone_type zt, zone_last = ZONE_NORMAL;
> +	enum zone_type zt;
>  
>  	/*
> -	 * 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.
> +	 * Check whether node_states[N_NORMAL_MEMORY] will be changed.
> +	 * If the memory to be offline is within the range
> +	 * [0..ZONE_NORMAL], and it is the last present memory there,
> +	 * the zones in that range will become empty after the offlining,
> +	 * thus we can determine that we need to clear the node from
> +	 * node_states[N_NORMAL_MEMORY].
>  	 */
> -	if (N_MEMORY == N_NORMAL_MEMORY)
> -		zone_last = ZONE_MOVABLE;
> -
> -	/*
> -	 * check whether node_states[N_NORMAL_MEMORY] will be changed.
> -	 * If the memory to be offline is in a zone of 0...zone_last,
> -	 * and it is the last present memory, 0...zone_last will
> -	 * become empty after offline , thus we can determind we will
> -	 * need to clear the node from node_states[N_NORMAL_MEMORY].
> -	 */
> -	for (zt = 0; zt <= zone_last; zt++)
> +	for (zt = 0; zt <= ZONE_NORMAL; zt++)
>  		present_pages += pgdat->node_zones[zt].present_pages;
> -	if (zone_idx(zone) <= zone_last && nr_pages >= present_pages)
> +	if (zone_idx(zone) <= ZONE_NORMAL && nr_pages >= present_pages)
>  		arg->status_change_nid_normal = zone_to_nid(zone);
>  	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.
> +	 * node_states[N_HIGH_MEMORY] contains nodes which
> +	 * have normal memory or high memory.
> +	 * Here we add the present_pages belonging to ZONE_HIGHMEM.
> +	 * If the zone is within the range of [0..ZONE_HIGHMEM), and
> +	 * we determine that the zones in that range become empty,
> +	 * we need to clear the node for N_HIGH_MEMORY.
>  	 */
> -	zone_last = ZONE_HIGHMEM;
> -	if (N_MEMORY == N_HIGH_MEMORY)
> -		zone_last = ZONE_MOVABLE;
> +	zt = ZONE_HIGHMEM;
> +	present_pages += pgdat->node_zones[zt].present_pages;
>  
> -	for (; zt <= zone_last; zt++)
> -		present_pages += pgdat->node_zones[zt].present_pages;
> -	if (zone_idx(zone) <= zone_last && nr_pages >= present_pages)
> +	if (zone_idx(zone) <= zt && nr_pages >= present_pages)
>  		arg->status_change_nid_high = zone_to_nid(zone);
>  	else
>  		arg->status_change_nid_high = -1;
> @@ -1542,18 +1527,18 @@ static void node_states_check_changes_offline(unsigned long nr_pages,
>  #endif
>  
>  	/*
> -	 * node_states[N_HIGH_MEMORY] contains nodes which have 0...ZONE_MOVABLE
> +	 * We have accounted the pages from [0..ZONE_NORMAL), and
> +	 * in case of CONFIG_HIGHMEM the pages from ZONE_HIGHMEM
> +	 * as well.
> +	 * Here we count the possible pages from ZONE_MOVABLE.
> +	 * If after having accounted all the pages, we see that the nr_pages
> +	 * to be offlined is over or equal to the accounted pages,
> +	 * we know that the node will become empty, and so, we can clear
> +	 * it for N_MEMORY as well.
>  	 */
> -	zone_last = ZONE_MOVABLE;
> +	zt = ZONE_MOVABLE;
> +	present_pages += pgdat->node_zones[zt].present_pages;
>  
> -	/*
> -	 * check whether node_states[N_HIGH_MEMORY] will be changed
> -	 * If we try to offline the last present @nr_pages from the node,
> -	 * we can determind we will need to clear the node from
> -	 * node_states[N_HIGH_MEMORY].
> -	 */
> -	for (; zt <= zone_last; zt++)
> -		present_pages += pgdat->node_zones[zt].present_pages;
>  	if (nr_pages >= present_pages)
>  		arg->status_change_nid = zone_to_nid(zone);
>  	else
> 


A couple nits:

I would simplify this a little, initialize all fields at the beginning

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

And remove all the elses, including ifdef else.

And remove:
        zt = ZONE_HIGHMEM;
        zt = ZONE_MOVABLE;

Use macros directly, lines are are still going to be under 80 chars.

Otherwise looks good.

Reviewed-by: Pavel Tatashin <pavel.tatashin@...rosoft.com>

Pavel