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Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Muchun Song , Oscar Salvador , Alexander Graf , David Matlack , David Rientjes , Jason Gunthorpe , Samiullah Khawaja , Vipin Sharma , Zhu Yanjun , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, kexec@lists.infradead.org Subject: Re: [RFC PATCH 01/10] kho: drop restriction on maximum page order In-Reply-To: (Pasha Tatashin's message of "Tue, 23 Dec 2025 12:59:05 -0500") References: <20251206230222.853493-1-pratyush@kernel.org> <20251206230222.853493-2-pratyush@kernel.org> Date: Mon, 29 Dec 2025 22:24:56 +0100 Message-ID: <86ikdp7zh3.fsf@kernel.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Stat-Signature: ni8854emtrn347zpdamopafyfo3ykk9j X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 70EBD40004 X-Rspam-User: X-HE-Tag: 1767043507-73436 X-HE-Meta: U2FsdGVkX1+CaspoFXQ4DBnafgEgc+4ptT/6oy7dj+r4MJjkwo1++qnjzsnGoOFi2X98xe0LddgAamXWEl7OpohLnhEYHBoCyn/RdAsAbXSbO8eJQhTS9R/yuzTPzj8RsUZirQyfwuhWUMR5Ma6n9fOrnBYx7vfBP70F6bvAcP/ZUjHf/SBP1BGWa8/23n2MK04cdC/G/qBvwlxvTZBfJw11hXHE2vRFrwhMunNWkY73B4zQVOKH6YAG+cNaCHir5JvjBmMeM1Qrj/YM+znwGQ2do1ZIFrkTR5TVusX/nru/ff8mx27BHCkCHpgkfqFlLUmEMW/B7Ys3X8Kp47fAMgd8a30GO3hR8VRfi+xcofg2XL8qTODOJe3WKEo6Vf9ia2je0ploR64mn8SGmj1Ud2Jab4b0qHYSU2+nF4ktVWgwBPXvOZj/J16f387s/Bru9+bqaVMSJTDuYcHVy5x9l+kbkR7cYsC4GgsKty0ZxJXSSj2T+GYU05tYbTo8Y4aceFBsKslqxuq1qoqprwBtuIEIUz+pdElwlq10H2cnMOU1FGfHP4bHGH0aaDCOEez/6GMaRhkxsLTFjaiaTzulQbl9X2b1MQXCKorywKGUciCrRXuelyvLz1qeThUkDd+JQqPgo9LQGpfFWQp6PHPH1XTpvWw6acskyORAfhHG8CiWtTbyFBI9AgMxrIEUEDlFQrppblgAsL06QLBksxYFb7dkk3zwK3gL54Aj+vLhycjAcQ4SMe5ZEC5yFgxiiX2nZopStdzHkCQuTyIK8tXQ+NSMROio8f+6kH6225s8soYw3H/T5f4XZ1cCApTmwUXOZfvxXyQuiG8HjCMHxQT/UXuJjpm07bTFStVPWfu3dEkzyyKGl1j+uUaJq3BEmCiqkCR/X6ZvhUyeSYyL2J8jt9EdPva1Ow0HvQ9guNCekoxznCHaRVzVQ+bnGlrjaU7fNp1AsqFJXq8M5IOiWOX yP3mLrOV XYjZY4/aY7rUIdcF6glxg5IalBIIPyPLeJlVFE9WzEDcHFL6i3o28+qPbC5Zz6u8VyDZ72x9+TX2RN6jAyrQm/D9C793LaHS4k6VjvvJ4BRzR2ynUh8YalhG3nQwOWG+1CuPhMlDPqteL4JJJRPT2T9St8Q1k8xNnzjdaHsbKQz0ROTpunGyPERd5ypbK6ZVeFAAmmt6XnMVp0a33oyLG8MlrvfJ7lkWXbpWk8Y/tkH+YBH8ZeFqRE04/8w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Dec 23 2025, Pasha Tatashin wrote: > On Sat, Dec 6, 2025 at 6:03=E2=80=AFPM Pratyush Yadav wrote: >> >> KHO currently restricts the maximum order of a restored page to the >> maximum order supported by the buddy allocator. While this works fine >> for much of the data passed across kexec, it is possible to have pages >> larger than MAX_PAGE_ORDER. >> >> For one, it is possible to get a larger order when using >> kho_preserve_pages() if the number of pages is large enough, since it >> tries to combine multiple aligned 0-order preservations into one higher >> order preservation. >> >> For another, upcoming support for hugepages can have gigantic hugepages >> being preserved over KHO. >> >> There is no real reason for this limit. The KHO preservation machinery >> can handle any page order. Remove this artificial restriction on max >> page order. > > Before removing this restriction, please also add a patch that makes > sure that the calculated order does not intermix memory from different > topologies. > > I suspect we need something like this in kho_preserve_pages(): > > order =3D min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn)); > /* Reduce order if the block spans across NUMA nodes. */ > while (order > 0) { > size_pfn =3D 1UL << order; > if (pfn_to_nid(pfn) =3D=3D pfn_to_nid(pfn + size_pfn - 1)) > break; > order--; > } ACK. Will do. And I think I will split it out of the HugeTLB series since I think this can be applied independently. --=20 Regards, Pratyush Yadav