[PATCH RFC] arm64/mm: Lift the cma address limit when CONFIG_DMA_NUMA_CMA=y

[Catalin Marinas]

On Wed, May 21, 2025 at 09:47:01AM +0800, Feng Tang wrote:
> When porting an cma related usage from x86_64 server to arm64 server,
> the "cma=4G" setup failed on arm64, and the reason is arm64 has 4G (32bit)
> address limit for cma reservation.
> 
> The limit is reasonable due to device DMA requirement, but for NUMA
> servers which have CONFIG_DMA_NUMA_CMA enabled, the limit is not required
> as that config already allows cma area to be reserved on different NUMA
> nodes whose memory very likely goes beyond 4G limit.
> 
> Lift the cma limit for platform with such configuration.

I don't think that's the right fix. Those devices that have a NUMA node
associated may be ok to address memory beyond 4GB. The default for
NUMA_NO_NODE devices is still dma_contiguous_default_area. I also don't
like to make such run-time decisions on the config option.

That said, maybe we should make the under-4G CMA allocation a best
effort. In the arch code, if that failed, attempt the allocation again
with a limit of 0 and maybe do a pr_notice() that CMA allocation in the
DMA zone failed.

Adding Robin in case he has a different view.

> Signed-off-by: Feng Tang <feng.tang at linux.alibaba.com>
> ---
>  arch/arm64/mm/init.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index b99bf3980fc6..661758678cc4 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -312,6 +312,7 @@ void __init arm64_memblock_init(void)
>  void __init bootmem_init(void)
>  {
>  	unsigned long min, max;
> +	phys_addr_t cma_limit;
>  
>  	min = PFN_UP(memblock_start_of_DRAM());
>  	max = PFN_DOWN(memblock_end_of_DRAM());
> @@ -343,8 +344,14 @@ void __init bootmem_init(void)
>  
>  	/*
>  	 * Reserve the CMA area after arm64_dma_phys_limit was initialised.
> +	 *
> +	 * When CONFIG_DMA_NUMA_CMA is enabled, system may have CMA reserved
> +	 * area in different NUMA nodes, which likely goes beyond the 32bit
> +	 * limit, thus use (PHYS_MASK+1) as cma limit.
>  	 */
> -	dma_contiguous_reserve(arm64_dma_phys_limit);
> +	cma_limit = IS_ENABLED(CONFIG_DMA_NUMA_CMA) ?
> +			(PHYS_MASK + 1) : arm64_dma_phys_limit;
> +	dma_contiguous_reserve(cma_limit);
>  
>  	/*
>  	 * request_standard_resources() depends on crashkernel's memory being
> -- 
> 2.39.5 (Apple Git-154)