linux-kernel - Re: [Patch BUGFIX] kcore: fix its wrong size on x86

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Message-ID: <4A367E6C.6030308@oracle.com>
Date:	Tue, 16 Jun 2009 01:01:32 +0800
From:	Tao Ma <tao.ma@...cle.com>
To:	ebiederm@...ssion.com
CC:	Amerigo Wang <xiyou.wangcong@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [Patch BUGFIX] kcore: fix its wrong size on x86_64



ebiederm@...ssion.com wrote:
> TaoMa <tao.ma@...cle.com> writes:
> 
>> ebiederm@...ssion.com wrote:
>>> Tao Ma <tao.ma@...cle.com> writes:
>>>
>>>   
>>>> Hi Amerigo,
>>>>
>>>> The wrong number I mean is 131941393240064.
>>>>
>>>> So do you think
>>>> [root@...t3 ~]# ls -l /proc/kcore
>>>> -r-------- 1 root root 131941393240064 Jun 15 13:39 /proc/kcore
>>>>
>>>> is better than
>>>>
>>>> [taoma@...t2 ~]$ ll /proc/kcore
>>>> -r-------- 1 root root 281474974617600 Jun 15 15:20 /proc/kcore
>>>> ?
>>>>
>>>> I don't think so.
>>>>
>>>> Actually the right result should look like
>>>>
>>>> [root@...t8 ~]# ls -l /proc/kcore
>>>> -r-------- 1 root root 5301604352 Jun 15 13:35 /proc/kcore
>>>>
>>>> And with your patch I can't get this number.
>>>>     
>>> Actually that value is the bug.  It has absolutely nothing
>>> to do with the offsets that are valid within /proc/kcore.
>>>
>>> Why do you prefer the smaller number?
>>>   
>> Amerigo said in the previous e-mail that " the man page for/proc/kcore is wrong,
>> its size can be more than the physical memory size, because it also contains
>> memory area of vmalloc(), vsyscall etc..."
>>
>> I have 4G memory, and 5301604352 is just a bit larger than 4G and looks sane. So
>> I misunderstand that this number is right.
> 
> It should also include the 32 Tebibyte range we have for vmalloc.  So
> a completely dense encoding would be a bit larger than 35184372088832
> bytes.  You can see that range in your readelf -l output.
> 
> Since the encoding is not dense the size actually comes to. 256TiB.
> Or roughly 281474976710656 bytes.
> 
>> But if it is also a bug, I am willing to test any of the new patch. ;)
> 
> Not in the sense that anything could go wrong.  Merely in the sense that
> we have a contradictory definition.  Which causes loads of confusion.
> 
> I am wondering if this difference in definition has caused any
> problems applications to fail or if this just started out as an
> observation of an anomaly?
I first noticed it when my el5 box refused to start kdump service and 
kexec said something like "Can't find kernel text map area from kcore". 
And then I found this number which looked a bit strange.
I also just have another x86 box and "ls -l /proc/kcore" shows:
-r-------- 1 root root 939528192 Jun 16 10:01 /proc/kcore
So I thought this may be a bug and started this thread.

Anyway, later I found that kexec's problem isn't related to this issue.
So maybe we can leave as-is.

regards,
Tao
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