linux-kernel - Re: [PATCH] mm/memory_hotplug: fix wrong casting for __remove

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Date:   Thu, 14 Sep 2017 11:43:10 -0400
From:   YASUAKI ISHIMATSU <yasu.isimatu@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        qiuxishi@...wei.com, arbab@...ux.vnet.ibm.com,
        Vlastimil Babka <vbabka@...e.cz>, yasu.isimatu@...il.com
Subject: Re: [PATCH] mm/memory_hotplug: fix wrong casting for
 __remove_section()

Hi Michal,

On 09/13/2017 01:59 AM, Michal Hocko wrote:
> On Tue 12-09-17 13:05:39, YASUAKI ISHIMATSU wrote:
>> Hi Michal,
>>
>> Thanks you for reviewing my patch.
>>
>> On 09/12/2017 08:49 AM, Michal Hocko wrote:
>>> On Fri 08-09-17 16:43:04, YASUAKI ISHIMATSU wrote:
>>>> __remove_section() calls __remove_zone() to shrink zone and pgdat.
>>>> But due to wrong castings, __remvoe_zone() cannot shrink zone
>>>> and pgdat correctly if pfn is over 0xffffffff.
>>>>
>>>> So the patch fixes the following 3 wrong castings.
>>>>
>>>>   1. find_smallest_section_pfn() returns 0 or start_pfn which defined
>>>>      as unsigned long. But the function always returns 32bit value
>>>>      since the function is defined as int.
>>>>
>>>>   2. find_biggest_section_pfn() returns 0 or pfn which defined as
>>>>      unsigned long. the function always returns 32bit value
>>>>      since the function is defined as int.
>>>
>>> this is indeed wrong. Pfns over would be really broken 15TB. Not that
>>> unrealistic these days
>>
>> Why 15TB?
> 
> 0xffffffff>>28
> 

Even thought I see your explanation, I cannot understand.

In my understanding, find_{smallest|biggest}_section_pfn() return integer.
So the functions always return 0x00000000 - 0xffffffff. Therefore if pfn is over
0xffffffff (under 16TB), then the function cannot work correctly.

What am I wrong?

Thanks,
Yasuaki Ishimatsu