linux-kernel - Re: [v2 1/5] arm64: kdump: reserve memory for crash dump kernel

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Message-ID: <5550659D.90006@linaro.org>
Date:	Mon, 11 May 2015 17:17:33 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Baoquan He <bhe@...hat.com>
CC:	catalin.marinas@....com, will.deacon@....com, vgoyal@...hat.com,
	hbabus@...ibm.com, geoff@...radead.org, broonie@...nel.org,
	david.griego@...aro.org, kexec@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 1/5] arm64: kdump: reserve memory for crash dump kernel

On 05/11/2015 04:54 PM, Baoquan He wrote:
> On 05/11/15 at 04:38pm, AKASHI Takahiro wrote:
>> Hi Baoquan,
>>
>> On 04/28/2015 06:19 PM, Baoquan He wrote:
>>>> +#ifdef CONFIG_CRASH_DUMP
>>>> +/*
>>>> + * reserve_elfcorehdr() - reserves memory for elf core header
>>>> + *
>>>> + * This function reserves memory area given in "elfcorehdr=" kernel command
>>>> + * line parameter. The memory reserved is used by a dump capture kernel to
>>>> + * identify the memory used by primary kernel.
>>>> + */
>>>
>>> Hi AKASHI,
>>>
>>> May I know why elfcorehdr need be reserved separately but not locate a
>>> memory region in crashkernel reserved region like all other ARCHs? Is
>>> there any special reason?
>>
>> I don't get your point, but arm as well as arm64 locates elfcorehdr
>> in a crash kernel's memory region.
>> See kexec/arch/arm{,64}/crashdump-arm{,64}.c in kexec-tools.
>>
>> And this region is reserved at boot time *on crash kernel* because we don't want
>> to corrupt it accidentally.
>> (After Mark's comment, we might better remove the mmu mapping for this region, too.)
>
>
> Sorry, I don't make myself clear.
>
> In this patch you reserve a separate memory region in 1st kernel to
> store elfcorehdr. I am wondering why you don't call add_buffer in
> kexec-tools directly. Like this you can get a region from reserved
> crashkernel region. Then you don't need reserve_elfcorehdr() to reserve
> memory for elfcorehdr specifically. Like other ARCHs do only one memory
> region is reserved in 1st kernel, that's crashkernel region.

I think that you misunderstand somewhat.
* Kexec-tools only locates/identifies a small region for elfcore header within crash kernel's
memory region while 1st kernel is running.
* the data in elfcore header is filled up by kexec_load system call on 1st kernel.
* 1st kernel doesn't reserve any region for elfcore header because the kernel
commandline parameters don't contains "elfcorehdr=" parameter, then elfcorehdr_size=0.
* Crash dump kernel does reserve the region, as I said, because we don't want to
corrupt the info in elfcore header accidentally while crash kernel is running.

Clear?

-Takahiro AKASHI

> Thanks
> Baoquan
>>
>>
>> Make sense?
>>
>> -Takahiro AKASHI
>>
>>> Thanks
>>> Baoquan
>>>
>>>> +static void __init reserve_elfcorehdr(void)
>>>> +{
>>>> +	if (!elfcorehdr_size)
>>>> +		return;
>>>> +
>>>> +	if (memblock_is_region_reserved(elfcorehdr_addr, elfcorehdr_size)) {
>>>> +		pr_warn("elfcorehdr reservation failed - memory is in use (0x%llx)\n",
>>>> +			elfcorehdr_addr);
>>>> +		return;
>>>> +	}
>>>> +
>>>> +	if (memblock_reserve(elfcorehdr_addr, elfcorehdr_size)) {
>>>> +		pr_warn("elfcorehdr reservation failed - out of memory\n");
>>>> +		return;
>>>> +	}
>>>> +
>>>> +	pr_info("Reserving %lldKB of memory at %lldMB for elfcorehdr\n",
>>>> +		elfcorehdr_size >> 10, elfcorehdr_addr >> 20);
>>>> +}
>>>> +#endif /* CONFIG_CRASH_DUMP */
>>>>   /*
>>>>    * Return the maximum physical address for ZONE_DMA (DMA_BIT_MASK(32)). It
>>>>    * currently assumes that for memory starting above 4G, 32-bit devices will
>>>> @@ -170,6 +247,13 @@ void __init arm64_memblock_init(void)
>>>>   		memblock_reserve(__virt_to_phys(initrd_start), initrd_end - initrd_start);
>>>>   #endif
>>>>
>>>> +#ifdef CONFIG_KEXEC
>>>> +	reserve_crashkernel(memory_limit);
>>>> +#endif
>>>> +#ifdef CONFIG_CRASH_DUMP
>>>> +	reserve_elfcorehdr();
>>>> +#endif
>>>> +
>>>>   	early_init_fdt_scan_reserved_mem();
>>>>
>>>>   	/* 4GB maximum for 32-bit only capable devices */
>>>> --
>>>> 1.7.9.5
>>>>
>>>> --
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