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

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Message-ID: <20190320084600.GG30433@dhcp22.suse.cz>
Date:   Wed, 20 Mar 2019 09:46:00 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        osalvador@...e.de, david@...hat.com, richard.weiyang@...il.com,
        rppt@...ux.ibm.com
Subject: Re: [PATCH] mm, memory_hotplug: Fix the wrong usage of N_HIGH_MEMORY

On Wed 20-03-19 16:07:32, Baoquan He 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.

N*MEMORY is confusing as hell but I am really curious whether we have
ZONE_DMA32 and ZONE_HIGMEM together?

That being said N.*MEMORY is intended to check for nodes rather than
zones so the patch looks good to me but I think the above explanation is
misleading and will add even more mud to the picture when somebody tries
to understand what the heck is going on here.

> Fixes: 8efe33f40f3e ("mm/memory_hotplug.c: simplify node_states_check_changes_online")
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  mm/memory_hotplug.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 6b05576fb4ec..09911d34a3be 100644
> --- 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
>  }
> -- 
> 2.17.2

-- 
Michal Hocko
SUSE Labs