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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 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 X-Rspam-User: X-Stat-Signature: qw1dwnjhiyiai4w5953qe1rc49hy6fgo X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: E098214000B X-HE-Tag: 1768495789-286371 X-HE-Meta: U2FsdGVkX1/bkj8HqR+GOrae9e3K3NQCD4mxaDtSInoLTWl1fhCI+kjabn2ldEmbdEqVzmYfPsAYQslf3c9ly0va0H0/65NuK5QuRhUyzH3rveWgHis5z0wc1ncMQ2SnQVHue85fGBzl6nQzVWmqkLfYnixzrZ6DujE5v9AtPhpyjiCkTv8ovEkPvpf2yw3j+XrB80fJRDNVJNQTF6hbda8hdiaZNL4VAtQogjU96ZeWhN5pKOl3KEH2DJCasDfrfzs1UmaxUGQwiMYMqafE+Hd0/5s7eb4mHKEjBxENYyJqHdQ225c5yGontS2rto3CxaZVbG8+tp3xA29sG8JIcjn9hbo55m62NXwy/6F8FjumbsI1xy+96IOOBu+DUxo3UK3Zy18/veOHuTjn2VwL0mgcRkEqk9HnoIR9aLb56uxxyYxQ9muHFSb+DTwrO5qCOvrf4kcIh1Gc2VyDU56mY2S6w4IgvTeHA1c4lYyfXApWjuA0Hsqm+sHTQ6+UVUcR94Iio+MYb10HjuxmdFJ3ecsYAGN1sIUqsIO964MuFQOIUtH2QpnpXnpVjf/wc1gCtHaZEhdwtFa0cl9fv2OzoEchuijeOGrnQCSuk2YmL0ioIkHvSulYTbTAK359nWMoOOFAatFO85AnBdasGnVk7Ei/JmLxDS/d4eWdZUgcUgUI1QHC2AzZu5i1F2GJeYg6uiuBFb8VTMk9mCLaYHzznS4m7Qpj0dCI5PLUukXxFzEOfZ75qZWixWoG5lUTGnQCtpBNUIxY8/Yf6OpIoo14xJT7K87z4lEXgdRZjA/fdHs6RIJ7grj8ecsSEDPMThm15H4AFF7gfQMkl6jWBAXg5DNq3uy05EqPFbxiQEqO9O/jP+iuMr0+ZVOo3nA2hyjnGP068SfEX2op6P0cGHIB3yQz7RJsD+sWYFPLtaYwMLueDyipT//lgprbxPslSV3IHB0oC8GGHt2268+0t7/ klmP3r7m v8nSTBDqhxrAN5yoWr5gVpjiSIBv2GpytlwEhPgkxUS0msCDfpeyGzA/D5+hly5/EkJpjBBX4LZspHdcN64XWi/1pdtzk0HigNdiHPDHLtE7f2pznxDao9QR61vF0Z4fS/d8FovOiaNXWu0ioDa4R34HJTlrDulZeiduKZT3hw3GxxEP1vJQRMPE89Y3nvdvqcdH833oz96rMTBtu7dWUwiog1cIcgMa0ZD+qJ4ZIr+IPGs9qleLZWm/9u++utkcr78MuWMj73A5WDaf8PGULwpRABw== 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 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