From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CCEA5EE2086 for ; Fri, 6 Feb 2026 10:35:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=OFixbIuU81W+j8PyH6JMuLZSGrIR0bhPFdJRt0BrLEA=; b=H9g/XNtBToCkRS Ot+CsNJpEvOp6hNB5kZzKNXClVnh5aKygrPz59nUyFGn6GoKTvOkVpU3lZ1Ejz5hjvvefGxdEyxu3 HcIjTSLj4IkOWeFO9mMLaamccrNyTfs/6upme8uY7yDbSjQxxcJmnBNf0/xxAoTbAeEofVulcdlNk yqksNSxvBAsGwG97w4OXzMh+gBCiLOOJ42L0D08P2s+cM/1Fhc98uuZzVaY5D8o1K7vyxy4t0rY75 kQTYhVVd8dtvoGswjo2J+7jLlDO2R7/OPLrnzqIRRnT9/reMgfP+E/Ls6r2YX3BjA1j9uhx99qQEi GKbceFzYTtzkxMiIrdBw==; 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charset="us-ascii"; Format="flowed" Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv