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

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Message-ID: <e5bc1884-ad75-63e3-ede5-e7ce88ec0f01@redhat.com>
Date:   Wed, 20 Mar 2019 10:11:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        osalvador@...e.de, richard.weiyang@...il.com, rppt@...ux.ibm.com
Subject: Re: [PATCH] mm, memory_hotplug: Fix the wrong usage of N_HIGH_MEMORY

On 20.03.19 10:06, Baoquan He wrote:
> 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.

Reviewed-by: David Hildenbrand <david@...hat.com>


-- 

Thanks,

David / dhildenb