linux-kernel - Re: [PATCH] mm, memory_hotplug: Fix the wrong usage of N_HIGH

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Message-Id: <20190320121209.5cd30d7b15f299df7d97d51e@linux-foundation.org>
Date:   Wed, 20 Mar 2019 12:12:09 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, osalvador@...e.de, mhocko@...e.com,
        david@...hat.com, richard.weiyang@...il.com, rppt@...ux.ibm.com,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm, memory_hotplug: Fix the wrong usage of
 N_HIGH_MEMORY

On Wed, 20 Mar 2019 16:07:32 +0800 Baoquan He <bhe@...hat.com> wrote:

> In function node_states_check_changes_online(), N_HIGH_MEMORY is used
> to substitute ZONE_HIGHMEM directly. This is not right. N_HIGH_MEMORY
> always has value '3' if CONFIG_HIGHMEM=y, while ZONE_HIGHMEM's value
> is not. It depends on whether CONFIG_ZONE_DMA/CONFIG_ZONE_DMA32 are
> enabled. Obviously it's not true for CONFIG_ZONE_DMA32 on 32bit system,
> and CONFIG_ZONE_DMA is also optional.
> 
> Replace it with ZONE_HIGHMEM.
> 
> Fixes: 8efe33f40f3e ("mm/memory_hotplug.c: simplify node_states_check_changes_online")

What are the runtime effects of this change?

> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -712,7 +712,7 @@ static void node_states_check_changes_online(unsigned long nr_pages,
>  	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))
> +	if (zone_idx(zone) <= ZONE_HIGHMEM && !node_state(nid, N_HIGH_MEMORY))
>  		arg->status_change_nid_high = nid;
>  #endif
>  }