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b=dl1EW8Bk/wk7n+ked6d5dTqvGdNDbGhl8PLkyO2JwRqCbK85aViLYdw68xiFFERHu YA2RE9D2nOCr1Mp+bpDoUvE7ZdJrHdTGzhVX5fZO+KivWZQwTWaecvAtWWkglQ75W2 kZWMNaiFgu7oOn0WetGjHSnQvEaxClLl30Ku/0aUVUK0XpoBfJmO3UmbZ+c8cUkd5X eYZwNkOZrx6cP3m1cYFkY+8bNZPHXr+yQRnCYctDNvif1JySmQdEaSpxX4nDRp+yOT HOvkRaiWNPUYYljW9aSgf8nHB/6/E/dzCRGl+D4kKv3Ul673G+YRA3K8/sUN+rb77H FV/Gto8aur1OQ== Message-ID: Date: Thu, 15 Jan 2026 18:41:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages To: Kiryl Shutsemau 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 References: <20260115144604.822702-1-kas@kernel.org> <20260115144604.822702-11-kas@kernel.org> <30ae1623-63f9-4729-9c19-9b0a9a0ae9f1@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-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 3BEDE1C0005 X-Rspam-User: X-Stat-Signature: kdwfsjjyfcizwdmw3gzwsigjqmfugo97 X-HE-Tag: 1768498912-136948 X-HE-Meta: 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 6RVJD/SW edyxLwUNrtFS4gzaavoM27P+k1fsdjAqPqQmXra9Ohw2Ngz919o9WpRjlnrrfFoRsol2M53UvW1T+r5Qx/0tYb/40/L5Z4bluCLjie1upcFp9FEH7fy7z0rEl0IYuqSLYW/rKVB37CsA8NhYEUhvjCk9968z4vC951NqilMQxqXWqkhisu7JlzMR/Run7V3gtFGJqq+8Bk6AJCUvDPS5PgF2BV4uQmI8o7Hlo1UzroZoKmw6KNj5QzcSm9Nc++4GDNZs3pA/pEPS/m+7nBH9Zycnix68tbYFvkn3D 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 18:23, 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. get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G) should evaluate to a constant by the compiler. See __builtin_constant_p handling in get_order(). If that is not working then we have to figure out why. Was this only a specific config in where you ran into compile-time problems? -- Cheers David