Re: [PATCH v3 2/2] arm64: mm: reserve CMA and crashkernel in ZONE_DMA32

[Jon Masters <jcm@jonmasters.org>]

On 3/22/21 2:34 PM, Jon Masters wrote:
> Hi Nicolas,
> 
> On 11/7/19 4:56 AM, Nicolas Saenz Julienne wrote:
>> With the introduction of ZONE_DMA in arm64 we moved the default CMA and
>> crashkernel reservation into that area. This caused a regression on big
>> machines that need big CMA and crashkernel reservations. Note that
>> ZONE_DMA is only 1GB big.
>>
>> Restore the previous behavior as the wide majority of devices are OK
>> with reserving these in ZONE_DMA32. The ones that need them in ZONE_DMA
>> will configure it explicitly.
>>
>> Reported-by: Qian Cai <cai@lca.pw>
>> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
>> ---
>>   arch/arm64/mm/init.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 580d1052ac34..8385d3c0733f 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -88,7 +88,7 @@ static void __init reserve_crashkernel(void)
>>       if (crash_base == 0) {
>>           /* Current arm64 boot protocol requires 2MB alignment */
>> -        crash_base = memblock_find_in_range(0, ARCH_LOW_ADDRESS_LIMIT,
>> +        crash_base = memblock_find_in_range(0, arm64_dma32_phys_limit,
>>                   crash_size, SZ_2M);
>>           if (crash_base == 0) {
>>               pr_warn("cannot allocate crashkernel (size:0x%llx)\n",
>> @@ -454,7 +454,7 @@ void __init arm64_memblock_init(void)
>>       high_memory = __va(memblock_end_of_DRAM() - 1) + 1;
>> -    dma_contiguous_reserve(arm64_dma_phys_limit ? : 
>> arm64_dma32_phys_limit);
>> +    dma_contiguous_reserve(arm64_dma32_phys_limit);
>>   }
>>   void __init bootmem_init(void)
> 
> Can we get a bit more of a backstory about what the regression was on 
> larger machines? If the 32-bit DMA region is too small, but the machine 
> otherwise has plenty of memory, the crashkernel reservation will fail. 
> Most e.g. enterprise users aren't going to respond to that situation by 
> determining the placement manually, they'll just not have a crashkernel.

Nevermind, looks like Catalin already changed this logic in Jan 2021 by 
removing arm64_dma32_phys_limit and I'm out of date.

Jon.

-- 
Computer Architect