linux-kernel - Re: [PATCH 1/5] arm64: kdump: reserve memory for crash dump kernel

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Date:	Thu, 09 Apr 2015 18:39:55 +0530
From:	Pratyush Anand <panand@...hat.com>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	catalin.marinas@....com, will.deacon@....com, vgoyal@...hat.com,
	hbabus@...ibm.com
CC:	linaro-kernel@...ts.linaro.org, geoff@...radead.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	broonie@...nel.org, david.griego@...aro.org,
	linux-arm-kernel@...ts.infradead.org,
	Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH 1/5] arm64: kdump: reserve memory for crash dump kernel

Hi Takahiro,

On Thursday 26 March 2015 01:58 PM, AKASHI Takahiro wrote:
> Crash dump kernel will access memory regions in system kernel via
> copy_oldmem_page(), which reads a page with ioremap'ing it assuming that
> such pages are not part of main memory of crash dump kernel.
> This is true under non-UEFI environment because kexec-tools modifies
> a device tree adding "usablemem" attributes to memory sections.
> Under UEFI, however, this is not true because UEFI remove memory sections
> in a device tree and export all the memory regions, even though they belong
> to system kernel.
>
> So we should add "mem=X[MG]" boot parameter to limit the meory size and
> avoid hitting the following assertion in ioremap():
> 	if (WARN_ON(pfn_valid(__phys_to_pfn(phys_addr))))
> 		return NULL;

Well I am using your updated kexec-tool which has support of automatic 
addition of "mem=" parameter. I found that this warning is still 
appearing and therefore another error about "Kdump: vmcore not initialized".

Memory address for which ioremap failed was almost at the top of 
crash_reserved_mem. So I modified kexec-tool [1] to accept user specific 
mem= parameter with a value lesser than physical location which was 
being remapped, however still the warning was there.

Further I noticed that there is no reserved memblock with nonzero 
memblock_region->size when early_mem -> memblock_enforce_memory_limit is 
called. Therefore this mem= param is not limiting memory location in my 
case.

I was just wondering, why do not we use ioremap_cache instead of ioremap 
in copy_oldmem_page?

~Pratyush

[1] 
https://github.com/pratyushanand/kexec-tools/commit/7dc38d587cb32d4522f6baf035d09eeaf71c5105
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