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imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=S6dc09FX; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1770374138; a=rsa-sha256; cv=none; b=dsoqtfW4YljvkZ8A0rPl7UUJpO25Zk8/gjM3i5o5OKOjj6igCr+NpUXKl6b+5AW1AyMFf+ at6g173p1h3jNnRUlMY/IE2s+zrG6o4qV+x078l9MT0Ui7vzDwD6jd9hh4ZfiYu7KIvhfg 2QNgqGpgOSAo2F8XhIPBktMV1itXdhs= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id E532860010; Fri, 6 Feb 2026 10:35:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9ADBFC116C6; Fri, 6 Feb 2026 10:35:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770374137; bh=4eHKEhmEK8xsEIAIBEaQ/Y5Lb5Y5nnEuAYz1Q4HZ+SU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=S6dc09FXA/oslyvgU5XYJDzbCDyPHsKVIty3YtaZStxu0DwL5tirjTKmYgh/T9eCa MlLAjkLZP2A+XpbtVzHQ5Hz16O2G1paaoIzPh2aoh42Kpm3eyzKze2jUxg0BBedXS2 s0zLAKQGkVIYs9iA7x4O3L+mjVW6FOgKEcEJkqoHCqohjarOqhC1D2k7FP3EF/jaRQ tzQLNwDlYLIgmWgUZgs4ly60PYVii+9ZTKLF/Xl5C74FJwKJ6isZZy1sDvMWNNjura pNWziuoPV3Q2aXliocKXSW2xjWG7kteH3CBWMUYoiMoM/1nrnYcrBfDVRyjSuuWQQ3 C+or9FiaofU8w== Message-ID: Date: Fri, 6 Feb 2026 11:35:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 16/17] hugetlb: Update vmemmap_dedup.rst To: Kiryl Shutsemau , Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-17-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Stat-Signature: dy789qy3cg31na5o1itcctmd7rx9qq44 X-Rspam-User: X-Rspamd-Queue-Id: B5CC51C0006 X-HE-Tag: 1770374138-437865 X-HE-Meta: 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 tvtPg+Bg xxeB1amwTLh1IFQcZ6ZXHar6bqc/WYP6isnZQMXqpgqQ8gociJbjoBa2/46S3E0/TiuTWd5WLtxrBk9luENPGQyBjNdxCMpF47odn7AQxjbcpBsNuB9Sgpd7vEclLJiMmNvliYSoHupVDUrpQCEp4IKyCdCwVWoQBrWLJ/dzSV+P1S7wiLqyVsDYt/2n2+DhURJYdRDGWkTaFBawhUlbGy62u9kwVA6dvzNdyr3KC6YkW8n3CeVuXJ9L1oUgZ3r1nnAp4PDoelrfDVr0X+t5pH9Da/S4BbqdKErCOnNkdn7mTZZBi7TG7w0lXiAa5vkmp9P15258mTU99x8w= 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 2/2/26 16:56, Kiryl Shutsemau wrote: > Update the documentation regarding vmemmap optimization for hugetlb to > reflect the changes in how the kernel maps the tail pages. > > Fake heads no longer exist. Remove their description. > > Signed-off-by: Kiryl Shutsemau > Reviewed-by: Muchun Song > --- > Documentation/mm/vmemmap_dedup.rst | 60 +++++++++++++----------------- > 1 file changed, 26 insertions(+), 34 deletions(-) > > diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst > index 1863d88d2dcb..fca9d0ce282a 100644 > --- a/Documentation/mm/vmemmap_dedup.rst > +++ b/Documentation/mm/vmemmap_dedup.rst > @@ -124,33 +124,35 @@ Here is how things look before optimization:: > | | > +-----------+ > > -The value of page->compound_info is the same for all tail pages. The first > -page of ``struct page`` (page 0) associated with the HugeTLB page contains the 4 > -``struct page`` necessary to describe the HugeTLB. The only use of the remaining > -pages of ``struct page`` (page 1 to page 7) is to point to page->compound_info. > -Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct page`` > -will be used for each HugeTLB page. This will allow us to free the remaining > -7 pages to the buddy allocator. > +The first page of ``struct page`` (page 0) associated with the HugeTLB page > +contains the 4 ``struct page`` necessary to describe the HugeTLB. The remaining > +pages of ``struct page`` (page 1 to page 7) are tail pages. > + > +The optimization is only applied when the size of the struct page is a power-of-2 > +In this case, all tail pages of the same order are identical. See > +compound_head(). This allows us to remap the tail pages of the vmemmap to a > +shared, read-only page. The head page is also remapped to a new page. This > +allows the original vmemmap pages to be freed. > > Here is how things look after remapping:: > > - HugeTLB struct pages(8 pages) page frame(8 pages) > - +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > - | | | 0 | -------------> | 0 | > - | | +-----------+ +-----------+ > - | | | 1 | ---------------^ ^ ^ ^ ^ ^ ^ > - | | +-----------+ | | | | | | > - | | | 2 | -----------------+ | | | | | > - | | +-----------+ | | | | | > - | | | 3 | -------------------+ | | | | > - | | +-----------+ | | | | > - | | | 4 | ---------------------+ | | | > - | PMD | +-----------+ | | | > - | level | | 5 | -----------------------+ | | > - | mapping | +-----------+ | | > - | | | 6 | -------------------------+ | > - | | +-----------+ | > - | | | 7 | ---------------------------+ > + HugeTLB struct pages(8 pages) page frame You could highlight that we allocate a new head page like "page frame (new)". Wasn't aware of that detail before reading your change above. Reviewed-by: David Hildenbrand (Arm) -- Cheers, David