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

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Date:   Fri, 15 Sep 2017 11:36:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     YASUAKI ISHIMATSU <yasu.isimatu@...il.com>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        qiuxishi@...wei.com, arbab@...ux.vnet.ibm.com,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/memory_hotplug: fix wrong casting for
 __remove_section()

On Thu 14-09-17 11:43:10, YASUAKI ISHIMATSU wrote:
> 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?

You are not wrong. We are talking about the same thing AFAICS. I was
just less precise...

-- 
Michal Hocko
SUSE Labs