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From: Nikita Yushchenko <nyushchenko@dev.rtsoft.ru>
Organization: RTsoft Software Development Center
To: Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH] kexec/powerpc: fix exporting memory limit
Date: Fri, 7 Mar 2014 08:38:05 +0400
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        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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> 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.