Re: [PATCH v5] dma-contiguous: add kconfig option to setup numa cma area if not configured explicitly

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 12.05.2026 10:55, Feng Tang wrote:
> There was a report on a multi-numa-nodes arm64 server that when IOMMU
> is disabled, the dma_alloc_coherent() function always returns memory
> from node 0 even for devices attaching to other nodes, while they can
> get local dma memory when IOMMU is on with the same API.
>
> The reason is, when IOMMU is disabled, the dma_alloc_coherent() will
> go the direct way and call dma_alloc_contiguous(). The system doesn't
> have any explicit cma setting (like per-numa cma), and only has a
> default 64MB cma reserved area (on node 0), where kernel will try
> first to allocate memory from.
>
> Robin Murphy suggested to setup pernuma cma or disable cma, which did
> solve the issue. While there is still concern that for customers
> which don't have much kernel knowledge, they could still suffer from
> this silently as some architectures enable cma area by default (not
> an issue for X86 though, which set CONFIG_CMA_SIZE_MBYTES to 0 by
> default) for most Linux distributions.
>
> One thought is to follow the current cma reserving policy for platform
> with 'CONFIG_DMA_NUMA_CMA=y', that if the numa cma (either the 'numa cma'
> or 'cma pernuma' method) is not explicitly configured, and the platform
> really has multiple NUMA nodes, set it up according to size of default
> 'dma_contiguous_default_area'. This way, the default behavior of
> platform with one NUMA node is kept unchanged (say embedded/small
> devices don't need to allocate extra memory), while the general dma
> locality is improved.
>
> Add a new bool kernel config CONFIG_CMA_SIZE_PERNUMA to control whether
> to enable it. Even when the config is enabled, user can still disable
> it by kernel-cmdline setting like "numa_cma=0:0" or "cma_pernuma=0".
>
> Reported-by: Changrong Chen <chenchangrong.ccr@alibaba-inc.com>
> Suggested-by: Ying Huang <ying.huang@linux.alibaba.com>
> Suggested-by: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Feng Tang <feng.tang@linux.alibaba.com>

Applied to dma-mapping-for-next. Thanks!

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland