linux-kernel - Re: [Patch] kexec: increase max of kexec segments and use dynamic allocation

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Message-ID: <m1lj90p8yp.fsf@fess.ebiederm.org>
Date:	Sat, 24 Jul 2010 19:54:22 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	huang ying <huang.ying.caritas@...il.com>
Cc:	Cong Wang <amwang@...hat.com>, linux-kernel@...r.kernel.org,
	nhorman@...hat.com, akpm@...ux-foundation.org
Subject: Re: [Patch] kexec: increase max of kexec segments and use dynamic  allocation

huang ying <huang.ying.caritas@...il.com> writes:

> On Thu, Jul 22, 2010 at 3:08 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Cong Wang <amwang@...hat.com> writes:
>>
>>> On 07/22/10 14:28, Eric W. Biederman wrote:
>>>> Amerigo Wang<amwang@...hat.com>  writes:
>>>>
>>>>> Currently KEXEC_SEGMENT_MAX is only 16 which is too small for machine with
>>>>> many memory ranges. Increase this hard limit to 1024 which is reasonably large,
>>>>> and change ->segment from a static array to a dynamically allocated memory.
>>>>
>>>> ???
>>>>
>>>> This should be about segments in the executable being loaded.  What
>>>> executable has one segment for each range of physical memory?
>>>>
>>>> Not that generalizing this is a bad idea but with a comment that
>>>> seems entirely wrong I am wondering what the problem really is.
>>>>
>>>
>>> Ah, I think Neil should explain this.
>>>
>>> He made a patch which includes many memory ranges, caused kexec
>>> fails to load the kernel. Increasing this limit and the corresponding
>>> one in kexec-tools fixes the problem. His patch is not in upstream
>>> kexec-tools, AFAIK.
>>>
>>> However, even if we don't consider that patch, isn't 16 too small too?
>>
>> Generally you just need one physical hunk for the code, maybe a second
>> for the initrd.
>>
>> It is perfectly fine to raise the number of segments as it doesn't
>> affect the ABI, but it wants a good explanation of what kind of weird
>> application wants to write to all over memory when it is loaded.
>
> kexec can be used to load not only the kernel images, but also more
> complex images such as hibernation image. So I think it is good to
> raise the number of segments.

Totally reasonable.

And in all fairness the patch does a good job of raising the limit.

However if that is the goal 1024 is probably a bit low as I believe
SGI has built machines with that many nodes.  Still after the patch
under discussion 1024 was only a limit in a header file so it can
be trivially changed.

Eric
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