linux-kernel - Re: [PATCH] kexec/powerpc: fix exporting memory limit

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Date:	Fri, 7 Mar 2014 08:38:05 +0400
From:	Nikita Yushchenko <nyushchenko@....rtsoft.ru>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	kexec@...ts.infradead.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Hari Bathini <hbathini@...ux.vnet.ibm.com>,
	Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, yadviga@....rtsoft.ru,
	lugovskoy@....rtsoft.ru, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kexec/powerpc: fix exporting memory limit

> On Thu, 2014-03-06 at 18:24 +0400, Nikita Yushchenko wrote:
> > When preparing dump-capturing kernel, kexec userspace tool needs to
> > know actual amount of memory used by the running kernel. This may
> > differ from extire available DRAM for a couple of reasons. To address
> > this issue, kdump kernel support code injects several attributes into
> > device tree that are later captured by userspace kexec tool via /proc
> > interface.
> >
> > One such attrubute is 'chosen/linux,memory_limit' that is used to pass
> > memory limit of the running kernel.
> >
> > This was initialized using kernel's 'memory_limit' variable, that is
> > set by early init code based on mem= kernel parameter and other
> > reasons.
> >
> > But there are cases when memory_limit variable does not contain proper
> > information. One such case is when !CONFIG_HIGHMEM kernel runs on
> > system with memory large enough not to fit into lowmem.
>
> Why doesn't the !CONFIG_HIGHMEM code update memory_limit to reflect
> reality.

I guess because memory_limit is used for ... well, memory limit, set by 
mem=. And for the rest memblock is used (and it *is* updated).

And code elsewhere does use memblock, see e.g. numa_enforce_memory_limit() 
in arch/powerpc/mm/numa.c

In MMU init (MMU_init() in arch/powerpc/mm/init_32.c -which is the point 
where final memory configuration is set) memblock, not memory_limit, is 
both used and updated.
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