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Fri, 16 Jan 2026 10:52:06 -0500 (EST) Date: Fri, 16 Jan 2026 15:52:05 +0000 From: Kiryl Shutsemau To: Muchun Song Cc: "David Hildenbrand (Red Hat)" , Andrew Morton , 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> <53980726-C7F0-4648-99E9-89E10645F2E7@linux.dev> 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: <53980726-C7F0-4648-99E9-89E10645F2E7@linux.dev> On Fri, Jan 16, 2026 at 10:38:02AM +0800, Muchun Song wrote: > > > > On Jan 16, 2026, at 01:23, Kiryl Shutsemau wrote: > > > > 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. > > How about allocate ->vmemmap_tails array dynamically? If sizeof of struct > page is not power of two, then we could optimize away this array. Besides, > the original MAX_FOLIO_ORDER could work as well. This is tricky. We need vmemmap_tails array to be around early, in hugetlb_vmemmap_init_early(). By the time, we don't have slab functional yet. I think getting the array compile-time is the best shot. -- Kiryl Shutsemau / Kirill A. Shutemov