Subject: Re: [PATCH] mm/memory_hotplug: fix wrong casting for
 __remove_section()
To: Michal Hocko <mhocko@kernel.org>
References: <51a59ec3-e7ba-2562-1917-036b8181092c@gmail.com>
 <20170912124952.uraxdt5bgl25zhf7@dhcp22.suse.cz>
 <587bdecd-2584-21be-94b8-61b427f1b0e8@gmail.com>
 <20170913055914.3npcxevhdwghcmdd@dhcp22.suse.cz>
Cc: linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
        qiuxishi@huawei.com, arbab@linux.vnet.ibm.com,
        Vlastimil Babka <vbabka@suse.cz>, yasu.isimatu@gmail.com
From: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>
Message-ID: <509197e7-135d-1304-76f1-32ae1fcbf223@gmail.com>
Date: Thu, 14 Sep 2017 11:43:10 -0400
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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