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Subject: Re: [PATCH v9 07/11] arm64: kexec_file: add crash dump support
From:   James Morse <james.morse@arm.com>
To:     AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc:     catalin.marinas@arm.com, will.deacon@arm.com, dhowells@redhat.com,
        vgoyal@redhat.com, herbert@gondor.apana.org.au,
        davem@davemloft.net, dyoung@redhat.com, bhe@redhat.com,
        arnd@arndb.de, ard.biesheuvel@linaro.org, bhsharma@redhat.com,
        kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
References: <20180425062629.29404-1-takahiro.akashi@linaro.org>
 <20180425062629.29404-8-takahiro.akashi@linaro.org>
 <f8d8305d-42f5-e1e2-20c4-733cb38cbb8d@arm.com>
Message-ID: <512cde7d-d84b-3623-cb09-a2d78acc11dc@arm.com>
Date:   Wed, 16 May 2018 11:06:02 +0100
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Hi Akashi,

On 15/05/18 18:11, James Morse wrote:
> On 25/04/18 07:26, AKASHI Takahiro wrote:
>> Enabling crash dump (kdump) includes
>> * prepare contents of ELF header of a core dump file, /proc/vmcore,
>>   using crash_prepare_elf64_headers(), and
>> * add two device tree properties, "linux,usable-memory-range" and
>>   "linux,elfcorehdr", which represent repsectively a memory range
>>   to be used by crash dump kernel and the header's location

>> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
>> index 37c0a9dc2e47..ec674f4d267c 100644
>> --- a/arch/arm64/kernel/machine_kexec_file.c
>> +++ b/arch/arm64/kernel/machine_kexec_file.c

>> +static struct crash_mem *get_crash_memory_ranges(void)
>> +{
>> +	unsigned int nr_ranges;
>> +	struct crash_mem *cmem;
>> +
>> +	nr_ranges = 1; /* for exclusion of crashkernel region */
>> +	walk_system_ram_res(0, -1, &nr_ranges, get_nr_ranges_callback);
>> +
>> +	cmem = vmalloc(sizeof(struct crash_mem) +
>> +			sizeof(struct crash_mem_range) * nr_ranges);
>> +	if (!cmem)
>> +		return NULL;
>> +
>> +	cmem->max_nr_ranges = nr_ranges;
>> +	cmem->nr_ranges = 0;
>> +	walk_system_ram_res(0, -1, cmem, add_mem_range_callback);
>> +
>> +	/* Exclude crashkernel region */
>> +	if (crash_exclude_mem_range(cmem, crashk_res.start, crashk_res.end)) {
>> +		vfree(cmem);
>> +		return NULL;
>> +	}
>> +
>> +	return cmem;
>> +}
> 
> Could this function be included in prepare_elf_headers() so that the alloc() and
> free() occur together.
> 
> 
>> +static int prepare_elf_headers(void **addr, unsigned long *sz)
>> +{
>> +	struct crash_mem *cmem;
>> +	int ret = 0;
>> +
>> +	cmem = get_crash_memory_ranges();
>> +	if (!cmem)
>> +		return -ENOMEM;
>> +
>> +	ret =  crash_prepare_elf64_headers(cmem, true, addr, sz);
>> +
>> +	vfree(cmem);
> 
>> +	return ret;
>> +}
> 
> All this is moving memory-range information from core-code's
> walk_system_ram_res() into core-code's struct crash_mem, and excluding
> crashk_res, which again is accessible to the core code.
> 
> It looks like this is duplicated in arch/x86 and arch/arm64 because arm64
> doesn't have a second 'crashk_low_res' region, and always wants elf64, instead
> of when IS_ENABLED(CONFIG_X86_64).

Thinking about it some more: don't we want to walk memblock here, not
walk_system_ram_res()? What we want is a list of not-nomap regions that the
kernel may have been using, to form part of vmcore.
walk_system_ram_res() is becoming a murkier list of maybe-nomap, maybe-reserved.

I think we should walk the same list here as we do in patch 4.


Thanks,

James