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b=GjLCFQjU5XtMIR8p2eUHF/UWAM3rz6xZtiteqH9mcF/Bbm0PvaCdhkYoFJLGQSAH6 NU+UqmQjnxZXYTuWhqjjm8pLO7oFBCYo+7rRTICVh4gAu67PGrqy90X1HVTMfio0E4 p35H3mRqvSxp47kRl96y94PYS6TM0FqgsBA89sR0EGpc7PfaGDp1yKJxz+SN67b8w9 GhHHdfFSgrTeOLdduvttmvYXLk4gsAiP9oUfNNhG9OwyzcAhwxyN2FGmmOMyfDjv7l WQU1ikAd8gcOb21b696ddPm2emWuXyL07t8Bxx9eFLZCAjBk2rh4RhpUAjz8Xklfjk Yrbw1tNNzGoVA== Message-ID: <6dd52f8b-7092-4d02-ba24-f8b55409b7b6@kernel.org> Date: Tue, 28 Apr 2026 09:52:15 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] dma-contiguous: setup default 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: <20260428060550.7167-1-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-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: C98FB4000A X-Stat-Signature: 4kxwgma16nu8zizz7uu7znwhwfkf75mj X-Rspam-User: X-HE-Tag: 1777362744-433531 X-HE-Meta: U2FsdGVkX18+C3bFYl04cn5ZqON7hAnCS4LRmPYN0U8ZhRnqOoaGacGey2Us2/ZvJkKcEFN6hJ0C84yajyY9BEtrR4jqCJ9JzrXEmdcdw3//JrzJu3EhJI4HcvXG06KCwf01o8j/Ss59md/8RXCwLC1Z+qP6WRrY4OM3aJhuxTvTXa9qYXPp7j6ieNMpGzHi7HyjxhR5cVM+3NP+d/yh757gLCOVrqG8Ecf8PtzGVkLDeZ+4hNLNoFGASAdEF00I0Gxn9SzNI1BGLbBMJpBFR7K2OxWChZXufe5LTrC5y+cYzMbOa8NEvdjx88LLqM1cEpfcn+iLQRKiQ3a/zNVPpo5OrY3MsYMgLyBz45exfis/c+XYCj6+2slMJejseSO/SMJaBqKAY4wl27vXlOIpF7CYnNyj5Wg3UdemgCyCko8FwsiujyH7Sy6/eRzNOmXAZ77TLGTNRuZrBEf2ScJDwpXoq5qAJmk30a8OeTzGqX7G0YME9AUuxMkfPbVEN0POH8o7oXyLsHuJtfMvxGeCiPPyIAwDmJ4ZEGez1rw7Kav/rWrV8cPSYVfug6jSc5WQOu2LOIm+rqfvfezwwA1lLMedSr+yZOW3L29MPrqo2DqMu7PHhDFhUF7DvcwKkug4Qmnw6fPwzt+RBg4S4T/ESedkHNVeCaGrY/BiEUn4ZF5+3V740hlBufqVpDfsQqLBUMqD90Kv03Cyh1vBIy8BtbQ2hYa6iAmFH54gNHPu+/FfWiz45HavaunUf29bURV+H98ON4pGhH++wuIhH9rkbAirsgN3YoOR4e9WsOQtt/bqrxA9wcR97p4szawhLp2//MqAQUcbUdld3b88ODRzMgrwYbPaNuFPYdqd+xLv0/Ljqysb54zD5BcJUrRkysoYjci9XGEhkex6yliCJuBy70EI9LpCqg6ST+saUxMBhjyO88EluS30CeAiidgtcFx8Xf6Ho3nACSRjUMByyfs nXk7Z17w jJenjj9hOlQuR0YOjudwoCIFi7/dNZ1aIH++P9FQAeWxy2cO+ES1RWVxzYY9Z7mTkujvrOSJRlf0x+ogVjYpJxomZ0C23yrNDpky4PB3H2f3Sop+1uHjAf4OYNAdBippF/xsg1+/FQzTZPkbxD8muZOuYV2ANS488hPbOIpc14IhDkDxGEkNb/hYrptaFqwHGeKkJ93eSAZRyEIjriR47VZmJk+UkczCVkss6/lP81RALkNZIEDcYPkuEWehDK5D5NuLZlZUmrcU4IATNEaNJxj20mnZ+G4wAVrFFlJTIJ6ixGbQiyJwLY5LY1grMYYCtWMFV1Qz07PXJjVIjBxyJZVlYvxYFoIfKBgWOh3rwjTzwaIaN+RmCFJopruvwvLJFCuIJ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/28/26 08:05, Feng Tang wrote: > There was a 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. > > Robin Murphy suggested to setup pernuma cma or disable cma, which did > solve the issue. That sounds like the obvious approach to me. > 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. Okay, so on x86 it is not silent, because they don't even have a default CMA area? > > 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. So, the kernel is configured to have a certain CONFIG_CMA_SIZE_MBYTES size, but you go ahead and multiply that by the number of nodes? Sounds wrong. The whole proposal here looks rather hacky. Wouldn't a default for e.g., pernuma_size_bytes make more sense, that users can then overwrite on the cmdline? > > To get the node info of cma area, add some helpr funciton and setup > in cma code. > > Reported-by: Changrong Chen > Suggested-by: Ying Huang > Suggested-by: Robin Murphy > Signed-off-by: Feng Tang > --- > Changelog: > > since v2: > * setup the numa cma are following default cma, while > skipping the node holds the default cma (Robin Murphy) > * add cma_get_node() help and related code > * add reporter info > > since v1: > * don't use the original way of adding alloc_pages_node() > before trying default cma node (Robin Murphy) > * setup default numa cma area if not configured (Ying Huang) > > v2: https://lore.kernel.org/lkml/20260423095243.14239-1-feng.tang@linux.alibaba.com/ > v1: https://lore.kernel.org/lkml/20260414090310.92055-1-feng.tang@linux.alibaba.com/ > > include/linux/cma.h | 1 + > kernel/dma/contiguous.c | 14 ++++++++++++-- > mm/cma.c | 11 ++++++++++- > 3 files changed, 23 insertions(+), 3 deletions(-) [...] > if (numa_cma_size[nid]) { > > cma = &dma_contiguous_numa_area[nid]; > @@ -255,8 +265,6 @@ void __init dma_contiguous_reserve(phys_addr_t limit) > phys_addr_t selected_limit = limit; > bool fixed = false; > > - dma_numa_cma_reserve(); > - > pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit); > > if (size_cmdline != -1) { > @@ -312,6 +320,8 @@ void __init dma_contiguous_reserve(phys_addr_t limit) > if (ret) > pr_warn("Couldn't queue default CMA region for heap creation."); > } > + > + dma_numa_cma_reserve(); > } > > void __weak > diff --git a/mm/cma.c b/mm/cma.c > index c7ca567f4c5c..3bbfafeaf6c1 100644 > --- a/mm/cma.c > +++ b/mm/cma.c > @@ -54,6 +54,11 @@ const char *cma_get_name(const struct cma *cma) > } > EXPORT_SYMBOL_GPL(cma_get_name); > > +extern int cma_get_nid(const struct cma *cma) > +{ > + return cma->nid; > +} Why do you have to store the nid instead of just looking it up from the base_pfn in here? Also, what is the expectation when the ranges would span different NIDs? (is that possible?) -- Cheers, David