linux-kernel - Re: [PATCH v3 2/2] arm64: mm: reserve CMA and crashkernel in ZONE

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Date:   Mon, 22 Mar 2021 14:34:38 -0400
From:   Jon Masters <jcm@...masters.org>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        catalin.marinas@....com, linux-kernel@...r.kernel.org
Cc:     Qian Cai <cai@....pw>, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/2] arm64: mm: reserve CMA and crashkernel in
 ZONE_DMA32

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@....pw>
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.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.

Jon.

-- 
Computer Architect