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Thu, 15 Jan 2026 12:23:28 -0500 (EST) Date: Thu, 15 Jan 2026 17:23:23 +0000 From: Kiryl Shutsemau To: "David Hildenbrand (Red Hat)" Cc: Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden , 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 Subject: Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages Message-ID: References: <20260115144604.822702-1-kas@kernel.org> <20260115144604.822702-11-kas@kernel.org> <30ae1623-63f9-4729-9c19-9b0a9a0ae9f1@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <30ae1623-63f9-4729-9c19-9b0a9a0ae9f1@kernel.org> On Thu, Jan 15, 2026 at 05:49:43PM +0100, David Hildenbrand (Red Hat) wrote: > 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 ? This doesn't change the resulting value: 1UL << 34 is 16GiB, 1UL << 30 is 1G. Subtract PAGE_SHIFT to get the order. The change allows the value to be used to define NR_VMEMMAP_TAILS which is used specify size of vmemmap_tails array. -- Kiryl Shutsemau / Kirill A. Shutemov