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

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Date:   Wed, 20 Mar 2019 20:53:21 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org,
        osalvador@...e.de, 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 20-03-19 12:12:09, Andrew Morton wrote:
> 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?

There shouldn't be none. The code is just messy. The newer changelog
should make it more clear I believe.

> > --- 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
> >  }
> 

-- 
Michal Hocko
SUSE Labs