From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6988833993; Thu, 15 Jan 2026 16:49:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768495789; cv=none; b=k0YGFDOYtt1hvIBp2oK3or+zAs+wfKrxpNwg9FotTVJ8VpfHi7wm4mena9T1sjvhHyAiAYQ0WKdemE3h6CoLd1mZdmr33R2Pscsglv6jc944EpQcRLDJhtwGOcYoBdpiOFIbQEu0xgCKKzQotxr7eiVhpIRa0fkh7aQqikipB/o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768495789; c=relaxed/simple; bh=IpZqoR5qAaoKI8w1KTi83tkryc1MfgEtvAY62yPtvPM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=sAsN6Ds2mYIsxpdLEECGONTIQ0LDzgzsD4Ish4145CKuEbt5xOdpFo5wKyn/6YtWyOLLhykx3q52Mgnwf3jiEJIJI4N3Gy5oaEz+fcLyCnLVErwHxoyEI2fpRhPjb03C6VBi1LqnCYxVm7wY707JI79c6RKTFuNbglf0ySersEc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HYrzVqfv; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HYrzVqfv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D4344C19421; Thu, 15 Jan 2026 16:49:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768495789; bh=IpZqoR5qAaoKI8w1KTi83tkryc1MfgEtvAY62yPtvPM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=HYrzVqfvqaeT4uBNVa/cbuS9BTam0Js1TKLqrPNiFk8aBczK7nXvKIHUpt65VkwCb CDZV9rG2yPmFspuBOwY5N4n8g2rELHh2qDWBR+qX/76buwnVT9k8Y3Ni1NqZYxue+B pYVDdGMSy9uLD6wepEFOLv3OWj2F3fkNWJmzR/KC08NmgYy3bw5ZFH/fIglxYcFE+A lQ0mbp9V/1+faH5BEvM1YMdO6r42QX0S9XSQ6rBRohcUvFCBujKZQ/3rlfPGvaCZ7L FIAwHMAqnawHANWDmMFFuYLPirZFywgHXtdfdiANmYCkMXoGEgnX1kc0Is/k7TbGpM AKPmMLws8a5uQ== Message-ID: <30ae1623-63f9-4729-9c19-9b0a9a0ae9f1@kernel.org> Date: Thu, 15 Jan 2026 17:49:43 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages 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 , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <20260115144604.822702-1-kas@kernel.org> <20260115144604.822702-11-kas@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/15/26 15:45, Kiryl Shutsemau wrote: > HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most > vmemmap pages for huge pages and remapping the freed range to a single > page containing the struct page metadata. > > With the new mask-based compound_info encoding (for power-of-2 struct > page sizes), all tail pages of the same order are now identical > regardless of which compound page they belong to. This means the tail > pages can be truly shared without fake heads. > > Allocate a single page of initialized tail struct pages per NUMA node > per order in the vmemmap_tails[] array in pglist_data. All huge pages > of that order on the node share this tail page, mapped read-only into > their vmemmap. The head page remains unique per huge page. > > This eliminates fake heads while maintaining the same memory savings, > and simplifies compound_head() by removing fake head detection. > > Signed-off-by: Kiryl Shutsemau > --- > include/linux/mmzone.h | 16 ++++++++++++++- > mm/hugetlb_vmemmap.c | 44 ++++++++++++++++++++++++++++++++++++++++-- > mm/sparse-vmemmap.c | 44 ++++++++++++++++++++++++++++++++++-------- > 3 files changed, 93 insertions(+), 11 deletions(-) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 322ed4c42cfc..2ee3eb610291 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -82,7 +82,11 @@ > * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect > * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit. > */ > -#define MAX_FOLIO_ORDER get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G) > +#ifdef CONFIG_64BIT > +#define MAX_FOLIO_ORDER (34 - PAGE_SHIFT) > +#else > +#define MAX_FOLIO_ORDER (30 - PAGE_SHIFT) > +#endif Where do these magic values stem from, and how do they related to the comment above that clearly spells out 16G vs. 1G ? -- Cheers David