linux-kernel - Re: [PATCH v6 1/2] dma-contiguous: provide the ability to reserve per-numa CMA

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Date:   Fri, 21 Aug 2020 09:47:17 +0100
From:   Will Deacon <will@...nel.org>
To:     Barry Song <song.bao.hua@...ilicon.com>
Cc:     hch@....de, m.szyprowski@...sung.com, robin.murphy@....com,
        ganapatrao.kulkarni@...ium.com, catalin.marinas@....com,
        iommu@...ts.linux-foundation.org, linuxarm@...wei.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        huangdaode@...wei.com,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Steve Capper <steve.capper@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v6 1/2] dma-contiguous: provide the ability to reserve
 per-numa CMA

On Fri, Aug 21, 2020 at 02:26:14PM +1200, Barry Song wrote:
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index bdc1f33fd3d1..3f33b89aeab5 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -599,6 +599,15 @@
>  			altogether. For more information, see
>  			include/linux/dma-contiguous.h
>  
> +	pernuma_cma=nn[MG]
> +			[ARM64,KNL]
> +			Sets the size of kernel per-numa memory area for
> +			contiguous memory allocations. A value of 0 disables
> +			per-numa CMA altogether. DMA users on node nid will
> +			first try to allocate buffer from the pernuma area
> +			which is located in node nid, if the allocation fails,
> +			they will fallback to the global default memory area.

What is the default behaviour if this option is not specified? Seems like
that should be mentioned here.

> diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
> index 847a9d1fa634..db7a37ed35eb 100644
> --- a/kernel/dma/Kconfig
> +++ b/kernel/dma/Kconfig
> @@ -118,6 +118,16 @@ config DMA_CMA
>  	  If unsure, say "n".
>  
>  if  DMA_CMA
> +
> +config DMA_PERNUMA_CMA
> +	bool "Enable separate DMA Contiguous Memory Area for each NUMA Node"

I don't understand the need for this config option. If you have DMA_DMA and
you have NUMA, why wouldn't you want this enabled?

> +	help
> +	  Enable this option to get pernuma CMA areas so that devices like
> +	  ARM64 SMMU can get local memory by DMA coherent APIs.
> +
> +	  You can set the size of pernuma CMA by specifying "pernuma_cma=size"
> +	  on the kernel's command line.
> +
>  comment "Default contiguous memory area size:"
>  
>  config CMA_SIZE_MBYTES
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index cff7e60968b9..89b95f10e56d 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -69,6 +69,19 @@ static int __init early_cma(char *p)
>  }
>  early_param("cma", early_cma);
>  
> +#ifdef CONFIG_DMA_PERNUMA_CMA
> +
> +static struct cma *dma_contiguous_pernuma_area[MAX_NUMNODES];
> +static phys_addr_t pernuma_size_bytes __initdata;
> +
> +static int __init early_pernuma_cma(char *p)
> +{
> +	pernuma_size_bytes = memparse(p, &p);
> +	return 0;
> +}
> +early_param("pernuma_cma", early_pernuma_cma);
> +#endif
> +
>  #ifdef CONFIG_CMA_SIZE_PERCENTAGE
>  
>  static phys_addr_t __init __maybe_unused cma_early_percent_memory(void)
> @@ -96,6 +109,34 @@ static inline __maybe_unused phys_addr_t cma_early_percent_memory(void)
>  
>  #endif
>  
> +#ifdef CONFIG_DMA_PERNUMA_CMA
> +void __init dma_pernuma_cma_reserve(void)
> +{
> +	int nid;
> +
> +	if (!pernuma_size_bytes)
> +		return;

If this is useful (I assume it is), then I think we should have a non-zero
default value, a bit like normal CMA does via CMA_SIZE_MBYTES.

> +	for_each_node_state(nid, N_ONLINE) {

for_each_online_node() {

> +		int ret;
> +		char name[20];

20?

Ah, wait, this is copy-pasta from hugetlb_cma_reserve(). Can you factor out
the common parts at all?

Will