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b=uuxg+xHjp/JYX/ccPTYilRbmu182EdYt08U/jdxjDVGgKnRvXUM15TARE3ZVW7Non CmhTBLpmO8p3NvXGPTPwKAoGfWNBuqaU1JyIj+dgx8H6w+g/E1zxanmS2VH7MRP/5D 06gIPeI+aYWECJL4OQ2XGa7wvxQ/cMkqrJU41IeaUTG1cZ5w45UVzOKtgXdMTbhH35 rmULiGEAl86uN+ij5+0OcrQ2xaOaHR2voRJrj2Dcztq/xzv3zLS4VEq5oHs8QHGAt0 jIgLqRjmFjmUuq4Rv7QqbN9sJAHFA3Qwl5pO8ExkLqEu4rddexdyUMJil6XbOSABm7 8QH5kIqlLyvRA== Message-ID: <1da07b4c-43a5-4eee-b7f5-0b96742df2a5@kernel.org> Date: Mon, 11 May 2026 15:47:39 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] dma-contiguous: add kconfig option to setup numa cma area if not configured explicitly To: Feng Tang , Marek Szyprowski , Robin Murphy , Ying Huang , Andrew Morton Cc: Lorenzo Stoakes , Liam.Howlett@oracle.com, Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, Christoph Hellwig , Catalin Marinas , Will Deacon , iommu@lists.linux.dev, linux-kernel@vger.kernel.org, Changrong Chen References: <20260509072543.69650-1-feng.tang@linux.alibaba.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 044ED100007 X-Rspamd-Server: rspam04 X-Stat-Signature: uh4m6ztsauek9q9ei1ofgtpqiokcfehn X-HE-Tag: 1778507264-755143 X-HE-Meta: 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 qOVHLL8K XD5ajhLcjl6+vmCowvcjGiQ2kPY9sHn/pqdpFuEVX76V9MZxAkpGRTn1wq7BVOytNMtcgf2PfazSJqU/JCXRB398bPyGH9l4Vu9XGFdgct4sV23xlZmKVInrWrHNqxzC+D0xhq87XLLZcnINs4OZhFtMt45bBFXFJFM+Y05qxGXaeI3HfycUzZ3k/erkxKvWgGMqgLZvywxmXVUbMHxoB2iz4jzdd4M/gwdC5jd9JT+FGMmNEiD8NEDqjtlrwcXfArdepLC3Q3P3qxhZofvVvGtaPFCBOtcX8Fqq5wUGvRO/aLT94jEDLjcICmZSOEI+TljvhjA1x0yMar2xC/UmhPd7lWIzn7SmsEuW9JyWuu9oBP9Q= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > Suggested-by: Ying Huang > Suggested-by: Robin Murphy > Signed-off-by: Feng Tang > --- > > 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