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Perez de Azpillaga" , "Muchun Song" Subject: Re: [PATCH 0/5] mm: hugetlb: add CMA support for PMD-sized page Date: Tue, 20 Oct 2026 15:46:49 +0800 Message-Id: <76a568ec-a23f-4a8b-ba14-e5a40083e1e4@bytedance.com> Content-Type: text/plain; charset=UTF-8 User-Agent: Mozilla Thunderbird X-Lms-Return-Path: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 662CA20005 X-Stat-Signature: tawszw6qstbj8dp4bbyq7scgnmrehg3d X-Rspam-User: X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-HE-Tag: 1792482432-338933 X-HE-Meta: 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 RYQcqo1U p6dUQss4Og68jlLS4W19GkfFT6j9c7FiXK5NTg9ZqHD/DFzupTIp/gDeObNnX4Xl8Pd/A6/4+vXA82xo8AMImmCf1Vl6STlYIkbU6yOg8bXpXOtPGM7zqQMGCn/c5n7qrllCJsVc6wi6mtfCit5OH+hEULiVVnCpWv8hBR9uLdcznla+ZR07/5a4vDAvEyYBvnjKK7KthNrncJhr+wfOejM4CJC6ANP81mONDdGMpdusvY/+ZYIVJWmCqiOiHLYg8ISmPXgcdF92jZlVR0gg4WNEPJg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 10/19/26 8:18 PM, David Hildenbrand (Arm) wrote: > On 10/19/26 11:17, Jose A. Perez de Azpillaga wrote: >> On 10/19/26 10:43 AM, Muchun Song wrote: >>> >>> >>>> >>>> >>>> wouldn't a shared pool still be sized and aligned for 1G? then the 2M >>>> headroom stays a multiple of 1G and the 1G-aligned reservation can still >>>> fail, which is what the series is trying to avoid. >>> >>> If we decide to use a shared pool, I think the current alignment strategy >>> should be adapted as well, it should be changed to align with the minimal >>> acquirement, like 2M. >>> >>> For example, if a user reserves 1G+2M through hugetlb_cma, then the memory >>> reserved by the pool is also 1G+2M, except that the starting address needs >>> to be aligned to the largest gigantic page (1G). Therefore, this pool is >>> suitable for both 1GB and 2MB allocation. The only problem I see with a >>> shared pool is that it may not support mixed-use scenarios very well. >>> >>> What do you think? >> >> makes sense to me. existing hugetlb_cma= users with WM pages would get >> mixed use without asking for it though. > > Yeah, my intuition tells me that having 1 allocation size per CMA area might be > cleaner. Agree. > > Assume you allocate 512 2 MiB hugetlb folios. You'd think that you would then be > able to allocate 1 GiB hugetlb folio less. But that's not guaranteed, as CMA > might just fallback to other areas fairly easily. > > If someone wants to convert from 1 GiB to 512 x 2MiB, I think the demotion > interface allows for that. For this series, I'd like to use a separate CMA pool for the newly supported PMD-sized pages, while keeping the existing sharing among gigantic sizes unchanged. If there are no objections, I'll update v2. Thanks, Qi