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Subject: Re: [PATCH v11 3/5] arm64: kdump: reimplement crashkernel=X
To:     Dave Young <dyoung@redhat.com>
References: <20200801130856.86625-1-chenzhou10@huawei.com>
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 <20200904031014.GA11869@dhcp-128-65.nay.redhat.com>
CC:     Catalin Marinas <catalin.marinas@arm.com>, <will@kernel.org>,
        <james.morse@arm.com>, <tglx@linutronix.de>, <mingo@redhat.com>,
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From:   chenzhou <chenzhou10@huawei.com>
Message-ID: <f4e0a246-0ca5-474b-8f39-c8299851d2b8@huawei.com>
Date:   Fri, 4 Sep 2020 12:02:05 +0800
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On 2020/9/4 11:10, Dave Young wrote:
> On 09/04/20 at 11:04am, Dave Young wrote:
>> On 09/03/20 at 07:26pm, chenzhou wrote:
>>> Hi Catalin,
>>>
>>>
>>> On 2020/9/3 1:09, Catalin Marinas wrote:
>>>> On Sat, Aug 01, 2020 at 09:08:54PM +0800, Chen Zhou wrote:
>>>>> There are following issues in arm64 kdump:
>>>>> 1. We use crashkernel=X to reserve crashkernel below 4G, which
>>>>> will fail when there is no enough low memory.
>>>>> 2. If reserving crashkernel above 4G, in this case, crash dump
>>>>> kernel will boot failure because there is no low memory available
>>>>> for allocation.
>>>>> 3. Since commit 1a8e1cef7603 ("arm64: use both ZONE_DMA and ZONE_DMA32"),
>>>>> if the memory reserved for crash dump kernel falled in ZONE_DMA32,
>>>>> the devices in crash dump kernel need to use ZONE_DMA will alloc
>>>>> fail.
>>>>>
>>>>> To solve these issues, change the behavior of crashkernel=X.
>>>>> crashkernel=X tries low allocation in ZONE_DMA, and fall back to
>>>>> high allocation if it fails.
>>>>>
>>>>> If requized size X is too large and leads to very little free memory
>>>>> in ZONE_DMA after low allocation, the system may not work normally.
>>>>> So add a threshold and go for high allocation directly if the required
>>>>> size is too large. The value of threshold is set as the half of
>>>>> the low memory.
>>>>>
>>>>> If crash_base is outside ZONE_DMA, try to allocate at least 256M in
>>>>> ZONE_DMA automatically. "crashkernel=Y,low" can be used to allocate
>>>>> specified size low memory.
>>>> Except for the threshold to keep zone ZONE_DMA memory,
>>>> reserve_crashkernel() looks very close to the x86 version. Shall we try
>>>> to make this generic as well? In the first instance, you could avoid the
>>>> threshold check if it takes an explicit ",high" option.
>>> Ok, i will try to do this.
>>>
>>> I look into the function reserve_crashkernel() of x86 and found the start address is
>>> CRASH_ALIGN in function memblock_find_in_range(), which is different with arm64.
>>>
>>> I don't figure out why is CRASH_ALIGN in x86, is there any specific reason?
>> Hmm, took another look at the option CONFIG_PHYSICAL_ALIGN
>> config PHYSICAL_ALIGN
>>         hex "Alignment value to which kernel should be aligned"
>>         default "0x200000"
>>         range 0x2000 0x1000000 if X86_32
>>         range 0x200000 0x1000000 if X86_64
>>
>> According to above, I think the 16M should come from the largest value
>> But the default value is 2M,  with smaller value reservation can have
>> more chance to succeed.
>>
>> It seems we still need arch specific CRASH_ALIGN, but the initial
>> version you added the #ifdef for different arches, can you move the
>> macro to arch specific headers?
> And just keep the x86 align value as is, I can try to change the x86
> value later to CONFIG_PHYSICAL_ALIGN, in this way this series can be
> cleaner.
Ok. I have no question about the value of macro CRASH_ALIGN,
instead the lower bound of memblock_find_in_range().

For x86, in reserve_crashkernel()，restrict the lower bound of the range to CRASH_ALIGN,
    ...
    crash_base = memblock_find_in_range(CRASH_ALIGN,
                                                CRASH_ADDR_LOW_MAX,
                                                crash_size, CRASH_ALIGN);
    ...
   
in reserve_crashkernel_low()，with no this restriction.
    ...
    low_base = memblock_find_in_range(0, 1ULL << 32, low_size, CRASH_ALIGN);
    ...

How about all making memblock_find_in_range() search from the start of memory?
If it is ok, i will do like this in the generic version.

Thanks,
Chen Zhou
>
>> Thanks
>> Dave
>
> .
>