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

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Message-ID: <20190320090624.GN18740@MiWiFi-R3L-srv>
Date:   Wed, 20 Mar 2019 17:06:24 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
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 03/20/19 at 09:46am, Michal Hocko wrote:
> 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?

Not sure. AFAIK, on x86_32 it can't be.

> 
> 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.

Yes, agree. I also thought this again after I sent out patch, feel log is not
good. As you said, they are value of enum node_states and enum zone_type
separately.

How about this:

~~~
In function node_states_check_changes_online(), N_HIGH_MEMORY is used
to substitute ZONE_HIGHMEM directly. This is not right. N_HIGH_MEMORY
is to mark the memory state of node. Here zone index is checked, which
should be compared with 'ZONE_HIGHMEM' accordingly.

Replace it with ZONE_HIGHMEM.
~~~
> 
> > 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