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

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Date:	Mon, 15 Jun 2009 12:48:13 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	TaoMa <tao.ma@...cle.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

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?

Eric


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