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Subject: Re: [PATCH 3/3 v3] x86/kdump: clean up all the code related to the
 backup region
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Dave Young <dyoung@redhat.com>
Cc:     linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, x86@kernel.org, bhe@redhat.com,
        jgross@suse.com, dhowells@redhat.com, Thomas.Lendacky@amd.com,
        vgoyal@redhat.com, kexec@lists.infradead.org
References: <20191012022140.19003-1-lijiang@redhat.com>
 <20191012022140.19003-4-lijiang@redhat.com>
 <87d0f22oi5.fsf@x220.int.ebiederm.org>
 <20191012121625.GA11587@dhcp-128-65.nay.redhat.com>
 <87zhi51ers.fsf@x220.int.ebiederm.org>
From:   lijiang <lijiang@redhat.com>
Message-ID: <72edff0b-9778-2e83-224b-7fe70dfb8d73@redhat.com>
Date:   Sun, 13 Oct 2019 17:36:28 +0800
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在 2019年10月13日 11:54, Eric W. Biederman 写道:
> Dave Young <dyoung@redhat.com> writes:
> 
>> Hi Eric,
>>
>> On 10/12/19 at 06:26am, Eric W. Biederman wrote:
>>> Lianbo Jiang <lijiang@redhat.com> writes:
>>>
>>>> When the crashkernel kernel command line option is specified, the
>>>> low 1MiB memory will always be reserved, which makes that the memory
>>>> allocated later won't fall into the low 1MiB area, thereby, it's not
>>>> necessary to create a backup region and also no need to copy the first
>>>> 640k content to a backup region.
>>>>
>>>> Currently, the code related to the backup region can be safely removed,
>>>> so lets clean up.
>>>>
>>>> Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
>>>> ---
>>>
>>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>>> index eb651fbde92a..cc5774fc84c0 100644
>>>> --- a/arch/x86/kernel/crash.c
>>>> +++ b/arch/x86/kernel/crash.c
>>>> @@ -173,8 +173,6 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>>>>  
>>>>  #ifdef CONFIG_KEXEC_FILE
>>>>  
>>>> -static unsigned long crash_zero_bytes;
>>>> -
>>>>  static int get_nr_ram_ranges_callback(struct resource *res, void *arg)
>>>>  {
>>>>  	unsigned int *nr_ranges = arg;
>>>> @@ -234,9 +232,15 @@ static int prepare_elf64_ram_headers_callback(struct resource *res, void *arg)
>>>>  {
>>>>  	struct crash_mem *cmem = arg;
>>>>  
>>>> -	cmem->ranges[cmem->nr_ranges].start = res->start;
>>>> -	cmem->ranges[cmem->nr_ranges].end = res->end;
>>>> -	cmem->nr_ranges++;
>>>> +	if (res->start >= SZ_1M) {
>>>> +		cmem->ranges[cmem->nr_ranges].start = res->start;
>>>> +		cmem->ranges[cmem->nr_ranges].end = res->end;
>>>> +		cmem->nr_ranges++;
>>>> +	} else if (res->end > SZ_1M) {
>>>> +		cmem->ranges[cmem->nr_ranges].start = SZ_1M;
>>>> +		cmem->ranges[cmem->nr_ranges].end = res->end;
>>>> +		cmem->nr_ranges++;
>>>> +	}
>>>
>>> What is going on with this chunk?  I can guess but this needs a clear
>>> comment.
>>
>> Indeed it needs some code comment, this is based on some offline
>> discussion.  cat /proc/vmcore will give a warning because ioremap is
>> mapping the system ram.
>>
>> We pass the first 1M to kdump kernel in e820 as system ram so that 2nd
>> kernel can use the low 1M memory because for example the trampoline
>> code.
>>
>>>
>>>>  
>>>>  	return 0;
>>>>  }
>>>
>>>> @@ -356,9 +337,12 @@ int crash_setup_memmap_entries(struct kimage *image, struct boot_params *params)
>>>>  	memset(&cmd, 0, sizeof(struct crash_memmap_data));
>>>>  	cmd.params = params;
>>>>  
>>>> -	/* Add first 640K segment */
>>>> -	ei.addr = image->arch.backup_src_start;
>>>> -	ei.size = image->arch.backup_src_sz;
>>>> +	/*
>>>> +	 * Add the low memory range[0x1000, SZ_1M], skip
>>>> +	 * the first zero page.
>>>> +	 */
>>>> +	ei.addr = PAGE_SIZE;
>>>> +	ei.size = SZ_1M - PAGE_SIZE;
>>>>  	ei.type = E820_TYPE_RAM;
>>>>  	add_e820_entry(params, &ei);
>>>
>>> Likewise here.  Why do we need a special case?
>>> Why the magic with PAGE_SIZE?
>>
>> Good catch, the zero page part is useless, I think no other special
>> reason, just assumed zero page is not usable, but it should be ok to
>> remove the special handling, just pass 0 - 1M is good enough.
> 
> But if we have stopped special casing the low 1M.  Why do we need a
> special case here at all?
> 
Here, need to pass the low memory range to kdump kernel, which will guarantee
the availability of low memory in kdump kernel, otherwise, kdump kernel won't
use the low memory region.

> If you need the special case it is almost certainly wrong to say you
> have ram above 640KiB and below 1MiB.  That is the legacy ROM and video
> MMIO area.
> 
> There is a reason the original code said 640KiB.
> 
Do you mean that the 640k region is good enough here instead of 1MiB?

Thanks.
Lianbo

> Eric
>