Date:   Wed, 29 Jul 2020 11:52:39 +0800
From:   chenzhou <chenzhou10@...wei.com>
To:     Catalin Marinas <catalin.marinas@....com>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <dyoung@...hat.com>,
        <bhe@...hat.com>, <will@...nel.org>, <james.morse@....com>,
        <robh+dt@...nel.org>, <arnd@...db.de>,
        <John.P.donnelly@...cle.com>, <prabhakar.pkin@...il.com>,
        <nsaenzjulienne@...e.de>, <corbet@....net>, <bhsharma@...hat.com>,
        <horms@...ge.net.au>, <guohanjun@...wei.com>,
        <xiexiuqi@...wei.com>, <huawei.libin@...wei.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kexec@...ts.infradead.org>, <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v10 4/5] arm64: kdump: fix kdump broken with ZONE_DMA
 reintroduced

Hi  Catalin,


On 2020/7/28 1:30, Catalin Marinas wrote:
> On Fri, Jul 03, 2020 at 11:58:15AM +0800, Chen Zhou wrote:
>> commit 1a8e1cef7603 ("arm64: use both ZONE_DMA and ZONE_DMA32")
>> broken the arm64 kdump. 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.
>>
>> This patch addressed the above issue based on "reserving crashkernel
>> above 4G". Originally, we reserve low memory below 4G, and now just need
>> to adjust memory limit to arm64_dma_phys_limit in reserve_crashkernel_low
>> if ZONE_DMA is enabled. That is, if there are devices need to use ZONE_DMA
>> in crash dump kernel, it is a good choice to use parameters
>> "crashkernel=X crashkernel=Y,low".
>>
>> Signed-off-by: Chen Zhou <chenzhou10@...wei.com>
>> ---
>>  kernel/crash_core.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
>> index a7580d291c37..e8ecbbc761a3 100644
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -320,6 +320,7 @@ int __init reserve_crashkernel_low(void)
>>  	unsigned long long base, low_base = 0, low_size = 0;
>>  	unsigned long total_low_mem;
>>  	int ret;
>> +	phys_addr_t crash_max = 1ULL << 32;
>>  
>>  	total_low_mem = memblock_mem_size(1UL << (32 - PAGE_SHIFT));
>>  
>> @@ -352,7 +353,11 @@ int __init reserve_crashkernel_low(void)
>>  			return 0;
>>  	}
>>  
>> -	low_base = memblock_find_in_range(0, 1ULL << 32, low_size, CRASH_ALIGN);
>> +#ifdef CONFIG_ARM64
>> +	if (IS_ENABLED(CONFIG_ZONE_DMA))
>> +		crash_max = arm64_dma_phys_limit;
>> +#endif
>> +	low_base = memblock_find_in_range(0, crash_max, low_size, CRASH_ALIGN);
>>  	if (!low_base) {
>>  		pr_err("Cannot reserve %ldMB crashkernel low memory, please try smaller size.\n",
>>  		       (unsigned long)(low_size >> 20));
> Given the number of #ifdefs we end up with in this function, I think
> it's better to simply copy to the code to arch/arm64 and tailor it
> accordingly.
>
> Anyway, there are two series solving slightly different issues with
> kdump reservations:
>
> 1. This series which relaxes the crashkernel= allocation to go anywhere
>    in the accessible space while having a dedicated crashkernel=X,low
>    option for ZONE_DMA.
>
> 2. Bhupesh's series [1] forcing crashkernel=X allocations only from
>    ZONE_DMA.
>
> For RPi4 support, we limited ZONE_DMA allocations to the 1st GB.
> Existing crashkernel= uses may no longer work, depending on where the
> allocation falls. Option (2) above is a quick fix assuming that the
> crashkernel reservation is small enough. What's a typical crashkernel
> option here? That series is probably more prone to reservation failures.
>
> Option (1), i.e. this series, doesn't solve the problem raised by
> Bhupesh unless one uses the crashkernel=X,low argument. It can actually
> make it worse even for ZONE_DMA32 since the allocation can go above 4G
> (assuming that we change the ZONE_DMA configuration to only limit it to
> 1GB on RPi4).
>
> I'm more inclined to keep the crashkernel= behaviour to ZONE_DMA
> allocations. If this is too small for typical kdump, we can look into
> expanding ZONE_DMA to 4G on non-RPi4 hardware (we had patches on the
> list). In addition, if Chen thinks allocations above 4G are still needed
> or if RPi4 needs a sufficiently large crashkernel=, I'd rather have a
> ",high" option to explicitly require such access.
Thanks for your reply and exhaustive explanation.

In our ARM servers, we need to to reserve a large chunk for kdump(512M or 1G),
there is no enough low memory. So we proposed this patch series
"support reserving crashkernel above 4G on arm64 kdump" In April 2019.

I introduce parameters "crashkernel=X,[high,low]" as x86_64 does in earlier versions.
Suggested by James, to simplify, we call reserve_crashkernel_low() at the beginning of
reserve_crashkernel() and then relax the arm64_dma32_phys_limit if reserve_crashkernel_low()
allocated something.
That is, just the parameter "crashkernel=X,low" is ok and i deleted "crashkernel=X,high".

After the ZONE_DMA introduced in December 2019, the issue occurred as you said above.
In fact, we didn't have RPi4 machine. Originally, i suggested to fix this based on this patch series
and used the dedicated option.

According to your clarify, for typical kdump, there are other solutions. In this case,
"keep the crashkernel= behaviour to ZONE_DMA allocations" looks much better.

How about like this:
1. For ZONE_DMA issue, use Bhupesh's solution, keep the crashkernel= behaviour to ZONE_DMA allocations.
2. For this patch series, make the reserve_crashkernel_low() to ZONE_DMA allocations.

Thanks,
Chen Zhou
> [1] http://lists.infradead.org/pipermail/kexec/2020-July/020777.html
>

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