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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/9/26 09:25, 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, set it up
> according to size of default 'dma_contiguous_default_area', while
> skipping the numa node where the 'dma_contiguous_default_area' lies
> in, 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).
> 
> 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".
> 
> To get the node info of cma area, add some helper function and setup
> in cma code.
> 
> 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>
> ---
> 
> In v4, the cma_get_nid() code is still kept, and if people really
> think it's not necessary, we can use the "nr_online_nodes() > 1" 
> instead as mentioned by Robin.

Yes, I'd hope we can get rid of that. It's rather nasty :)

-- 
Cheers,

David