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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777661507; bh=SgxH2HyW4oS4NiVwuhtY5isslD3L87GImOQpAVBRjl0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Gr1siNtIvuhyNyF2+IHVVnB28aTGvbbySnh6LT3C+b09uEvVEEWITbXV8+ZmvAZ8j MwgF2miTZ91k+herq6g2XJme7fFCLUKOkKJMXRwa9MrCLEnIn2TSyEWYQQj1zyZLAk GCl3Xn0C8uCd5FoufYENHyxiSCW2F3mgUs7trbHL5cVfZ9hR7yFG/S4YFPpFsjb95H xgqbQd++isSyUcIOs4OEFALkpiJZ7q+fqc8j/zlDY/apKlN1wXi4CyaA6Topa/ezB7 QWf28ujz3I6rZKVgHmSFroWplJih8A9zXVCV7vWpCLJunYVX+Cp6Cb0w7BabNvw7A5 UiXF+fR5FmisQ== Message-ID: Date: Fri, 1 May 2026 20:51:39 +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 Cc: Marek Szyprowski , Robin Murphy , Ying Huang , Andrew Morton , 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> <6dd52f8b-7092-4d02-ba24-f8b55409b7b6@kernel.org> 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-Stat-Signature: 7cmgjwi5yjzfft9jergzisuky36xrfkh X-Rspam-User: X-Rspamd-Queue-Id: 2100510000A X-Rspamd-Server: rspam07 X-HE-Tag: 1777661507-847498 X-HE-Meta: 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 ZwUa2eh8 7Mnz+Chxmbxsse6zNETmhLm5WC+ZGMUmx8mHSOvF1HmU9o7j25QyHIxtGMxIYGAB7PePUCzzEeSZFt+OV/hw3jkhjNQMg9cU1mTxgfH+7qMf96yHz0LfAaOXymUOZVC3JG/kVdjNnlfhdyMe34u2KRQgo8iFGgN1nzRbrdjNe1JUD40YSjgWEbeQxcH849SvH5WPgvAPsovYhEuY8VAnVTX8grh63G21VLcbKf9qDFs8ti/1OWQcuJw/Gsb5dyief64Q/ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/28/26 10:37, Feng Tang wrote: > Hi David, Hi! [...] >> >> Okay, so on x86 it is not silent, because they don't even have a default CMA area? > > Right for default kernel configs. > > In kernel/dma/Kconfig: > > config CMA_SIZE_MBYTES > int "Size in Mega Bytes" > depends on !CMA_SIZE_SEL_PERCENTAGE > default 0 if X86 > default 16 > > config CMA_SIZE_PERCENTAGE > int "Percentage of total memory" > depends on !CMA_SIZE_SEL_MBYTES > default 0 if X86 > default 10 > >>> >>> 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. > > Yes. I thought about that, and didn't have good solution, and used this > given it's on a multi-numa-node machine, which may not be too bad > regarding memory usage. It sounds wrong given the existing config options. > > Robin did concern about the memory usage for embedded/small devices in > v2 review, and we change to v3 to not affect them. > >> >> The whole proposal here looks rather hacky. > > I agree :) > >> Wouldn't a default for e.g., pernuma_size_bytes make more sense, that users can >> then overwrite on the cmdline? > > This sounds good to me, if no objection from others. Maybe default 64MB > or more. One good part is, all these setup is under protection of > CONFIG_DMA_NUMA_CMA. I cannot do the heavy thinking here because -EBUSY, but maybe you want a config option similar to CMA_SIZE_MBYTES/CMA_SIZE_PERCENTAGE that either controls that these sizes will be split over NUMA nodes, or another one, that sets the default for pernuma. [...] >>> +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? > > My thought was 'struct cma' already have 'nid' member, and when CONFIG_NUMA=y, > it may be useful to save the 'nid' info instead of NUMA_NO_NODE for the default > cma area (cmdline like cma=XXG@YYG could make it on different node) Ah, yeah. It's a bit nasty that we have to handle the default area like that. Another sign that we probably shouldn't deal with the default area :) > >> >> Also, what is the expectation when the ranges would span different NIDs? (is >> that possible?) > > Per my understanding, it won't. There is a cma_validate_zones() to prevent it > from crossing zones. It's a bit confusing, because it ignores other nids. -- Cheers, David