Re: [PATCH v1] dma-contiguous: try local node first for dma_alloc_contiguous()

[Robin Murphy <robin.murphy@arm.com>]

On 14/04/2026 10:03 am, Feng Tang wrote:
> There was a bug report on a multi-numa-nodes ARM 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.
> 
> Make the dma allocation more locality friendly by trying first the
> numa aware allocation alloc_pages_node() before falling back to the
> reserved cma area.
> 
> One more thought is to check the node of the reserved cma area and
> only call alloc_pages_nodes() when it isn't the same node that the
> device attaches to.

Frankly, no. We literally have a feature specifically created for this 
use-case already - if you care about NUMA locality of DMA allocations 
then enable DMA_NUMA_CMA. Or if you don't need CMA at all, then 
disabling CMA would be even better for the overall performance of your 
system.

Thanks,
Robin.

> Signed-off-by: Feng Tang <feng.tang@linux.alibaba.com>
> ---
>   kernel/dma/contiguous.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index c56004d314dc..0180f40f094e 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -371,8 +371,9 @@ static struct page *cma_alloc_aligned(struct cma *cma, size_t size, gfp_t gfp)
>    */
>   struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
>   {
> -#ifdef CONFIG_DMA_NUMA_CMA
> +#ifdef CONFIG_NUMA
>   	int nid = dev_to_node(dev);
> +	struct page *page;
>   #endif
>   
>   	/* CMA can be used only in the context which permits sleeping */
> @@ -386,7 +387,6 @@ struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
>   #ifdef CONFIG_DMA_NUMA_CMA
>   	if (nid != NUMA_NO_NODE && !(gfp & (GFP_DMA | GFP_DMA32))) {
>   		struct cma *cma = dma_contiguous_pernuma_area[nid];
> -		struct page *page;
>   
>   		if (cma) {
>   			page = cma_alloc_aligned(cma, size, gfp);
> @@ -402,6 +402,14 @@ struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
>   		}
>   	}
>   #endif
> +
> +#ifdef CONFIG_NUMA
> +	/* Try first to allocate memory on the same node as the device */
> +	page = alloc_pages_node(nid, gfp, get_order(size));
> +	if (page)
> +		return page;
> +#endif
> +
>   	if (!dma_contiguous_default_area)
>   		return NULL;
>