Re: [PATCH] arm64: mm: fix zone_dma_limit calculation

[Baruch Siach <baruch@tkos.co.il>]

Hi Yang,

On Mon, Nov 25 2024, Yang Shi wrote:
> The commit ba0fb44aed47 ("dma-mapping: replace zone_dma_bits by
> zone_dma_limit") changed how zone_dma_limit was calculated.  Now it
> returns the memsize limit in IORT or device tree instead of U32_MAX if
> the memsize limit is greater than U32_MAX.

Can you give a concrete example of memory layout and dma-ranges that
demonstrates this issue?

> This resulted in DMA allocations may use GFP_DMA even though the devices
> don't require it.  It caused regression on our two sockets systems due
> to excessive remote memory access.

That is, DMA zone used to cover all memory before commit ba0fb44aed47,
but now DMA zone is limited to the smallest dma-ranges. Is that correct?

Thanks,
baruch

> Fixes: ba0fb44aed47 ("dma-mapping: replace zone_dma_bits by zone_dma_limit")
> Cc: <stable@vger.kernel.org>    [6.12+]
> Reported-by: Yutang Jiang <jiangyutang@os.amperecomputing.com>
> Tested-by: Yutang Jiang <jiangyutang@os.amperecomputing.com>
> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
> ---
>  arch/arm64/mm/init.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index d21f67d67cf5..ccdef53872a0 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -117,15 +117,6 @@ static void __init arch_reserve_crashkernel(void)
>  
>  static phys_addr_t __init max_zone_phys(phys_addr_t zone_limit)
>  {
> -	/**
> -	 * Information we get from firmware (e.g. DT dma-ranges) describe DMA
> -	 * bus constraints. Devices using DMA might have their own limitations.
> -	 * Some of them rely on DMA zone in low 32-bit memory. Keep low RAM
> -	 * DMA zone on platforms that have RAM there.
> -	 */
> -	if (memblock_start_of_DRAM() < U32_MAX)
> -		zone_limit = min(zone_limit, U32_MAX);
> -
>  	return min(zone_limit, memblock_end_of_DRAM() - 1) + 1;
>  }
>  
> @@ -141,6 +132,14 @@ static void __init zone_sizes_init(void)
>  	acpi_zone_dma_limit = acpi_iort_dma_get_max_cpu_address();
>  	dt_zone_dma_limit = of_dma_get_max_cpu_address(NULL);
>  	zone_dma_limit = min(dt_zone_dma_limit, acpi_zone_dma_limit);
> +	/*
> +	 * Information we get from firmware (e.g. DT dma-ranges) describe DMA
> +	 * bus constraints. Devices using DMA might have their own limitations.
> +	 * Some of them rely on DMA zone in low 32-bit memory. Keep low RAM
> +	 * DMA zone on platforms that have RAM there.
> +	 */
> +	if (memblock_start_of_DRAM() < U32_MAX)
> +		zone_dma_limit = min(zone_dma_limit, U32_MAX);
>  	arm64_dma_phys_limit = max_zone_phys(zone_dma_limit);
>  	max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);
>  #endif

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