Re: [PATCH v14 02/11] x86: kdump: make the lower bound of crash kernel reservation consistent

[chenzhou]

On 2021/2/25 23:44, Baoquan He wrote:
> On 02/25/21 at 02:42pm, Catalin Marinas wrote:
>> On Thu, Feb 25, 2021 at 03:08:46PM +0800, Baoquan He wrote:
>>> On 02/24/21 at 02:35pm, Catalin Marinas wrote:
>>>> On Sat, Jan 30, 2021 at 03:10:16PM +0800, Chen Zhou wrote:
>>>>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>>>>> index da769845597d..27470479e4a3 100644
>>>>> --- a/arch/x86/kernel/setup.c
>>>>> +++ b/arch/x86/kernel/setup.c
>>>>> @@ -439,7 +439,8 @@ static int __init reserve_crashkernel_low(void)
>>>>>  			return 0;
>>>>>  	}
>>>>>  
>>>>> -	low_base = memblock_phys_alloc_range(low_size, CRASH_ALIGN, 0, CRASH_ADDR_LOW_MAX);
>>>>> +	low_base = memblock_phys_alloc_range(low_size, CRASH_ALIGN, CRASH_ALIGN,
>>>>> +			CRASH_ADDR_LOW_MAX);
>>>>>  	if (!low_base) {
>>>>>  		pr_err("Cannot reserve %ldMB crashkernel low memory, please try smaller size.\n",
>>>>>  		       (unsigned long)(low_size >> 20));
>>>> Is there any reason why the lower bound can't be 0 in all low cases
>>>> here? (Sorry if it's been already discussed, I lost track)
>>> Seems like a good question.
>>>
>>> This reserve_crashkernel_low(), paired with reserve_crashkernel_high(), is
>>> used to reserve memory under 4G so that kdump kernel owns memory for dma
>>> buffer allocation. In that case, kernel usually is loaded in high
>>> memory. In x86_64, kernel loading need be aligned to 16M because of
>>> CONFIG_PHYSICAL_START, please see commit 32105f7fd8faa7b ("x86: find
>>> offset for crashkernel reservation automatically"). But for crashkernel
>>> low memory, there seems to be no reason to ask for 16M alignment, if
>>> it's taken as dma buffer memory.
>>>
>>> So we can make a different alignment for low memory only, e.g 2M. But
>>> 16M alignment consistent with crashkernel,high is also fine to me. The
>>> only affect is smaller alignment can increase the possibility of
>>> crashkernel low reservation.
>> I don't mind the 16M alignment in both low and high base. But is there
>> any reason that the lower bound (third argument) cannot be 0 in both
>> reserve_crashkernel() (the low attempt) and reserve_crashkernel_low()
>> cases? The comment in reserve_crashkernel() only talks about the 4G
>> upper bound but not why we need a 16M lower bound.
> Ah, sorry, I must have mixed this one with the alignment of fixed
> memory region reservation in patch 1 when considering comments.
>
> Hmm, in x86 we always have memory reserved in low 1M, lower bound
> being 0 or 16M (kernel alignment) doesn't make difference on crashkernel
> low reservation. But for crashkernel reservation, the reason should be
> kernel loading alignment being 16M, please see commit 32105f7fd8faa7b
> ("x86: find offset for crashkernel reservation automatically").
Sorry, i didn't mention in the commit message about this.

We discussed about this and the CRASH_ALIGN sounds better, so just use CRASH_ALIGN.
https://lkml.org/lkml/2020/9/4/82

Thanks,
Chen Zhou
>
> So, for crashkernel low, keeping lower bound as 0 looks good to me, the
> only reason is just as patch log tells. And it can skip the unnecessary
> memblock searching under 16M since it will always fail, even though it
> won't matter much. Or changing it to CRASH_ALIGN as this patch is doing,
> and adding code comment, is also fine to me.
>
> Thanks
> Baoquan
>
> .
>