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b=RRylpIw2eIVb7L9jiz2iaEGM/RV33gOA0TX/2UtNgBjLDL0za8hdtwl+ym1TAAdYk rO/D1H+L3G1jrcZpLbeEc2exebBJs2fRf1RQSnrbK92Ngemue+WX+mZAgPQwylYlg7 rrtziz7OHiiUjLsV2PO6Yt6gNFCMroC3kGBcfKE8WHHNOqhHgy1Zpflx1aZ3o4n18g 2SQZtJwZ3EMpYh5DzRdtBwa4OzrSQGhQgJB35B1KNH2DyUN4F/07GxjVNpC7KsxmEa xO/vDMwFbDZ32X3ASDl4Nm10wMbRwhWHgoPD7VFx16hAd1yqWX/C/XUKbRXao8DJj0 E3S68YKLGpmGg== Message-ID: Date: Mon, 16 Mar 2026 18:01:51 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv7 12/18] mm/hugetlb: Remove fake head pages To: "Kiryl Shutsemau (Meta)" , 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 , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260227194302.274384-1-kas@kernel.org> <20260227194302.274384-13-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 55CB6A0002 X-Stat-Signature: ku4uhndxp6hji3e987szmk8nxru6qnjy X-Rspam-User: X-HE-Tag: 1773680520-991271 X-HE-Meta: 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 8uWri5FZ S4ZO1LPxMRGAriJzPmeGC7SP8QRPo+E1u0b13aW1E6NNQBW0cFefJM4EtdQkNReF7Huy24YEVDAdzG7nyy3MIB2v1CSSmrirygRvN+7WpVicrKnMj0aaqNa73m3zsV92x+D+LXC3kpRLQqG8Hdwio0fbx/cmYZ/OY3UOteeI/iasDBrGMcxghRfFNNoTaalXLVsH4MU4g7IMJo7XO7wIEUHyH6QVkhSCmbiXTr+wxjLViNdiNX0rgDwMvFVEwNa2aNNJV/7+gllTug/vRzc1uALuU6A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/27/26 20:42, Kiryl Shutsemau (Meta) wrote: > From: Kiryl Shutsemau > > 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 zone > per order in the vmemmap_tails[] array in struct zone. All huge pages of > that order in the zone share this tail page, mapped read-only into their > vmemmap. The head page remains unique per huge page. > > Redefine MAX_FOLIO_ORDER using ilog2(). The define has to produce a > compile-constant as it is used to specify vmemmap_tail array size. > For some reason, compiler is not able to solve get_order() at > compile-time, but ilog2() works. > > Avoid PUD_ORDER to define MAX_FOLIO_ORDER as it adds dependency to > which generates hard-to-break include loop. > > This eliminates fake heads while maintaining the same memory savings, > and simplifies compound_head() by removing fake head detection. > > Signed-off-by: Kiryl Shutsemau > --- [...] > > +static struct zone *pfn_to_zone(unsigned nid, unsigned long pfn) I'd make it clearer that this some hugetlb helper, not just some page_zone(pfn_to_page()). Also, it's only really valid during boot, where we cannot have overlapping zones yet (hopefully IIRC). static struct zone *hugetlb_early_pfn_to_zone(unsigned nid, unsigned long pfn) or sth. like that (throwing in an __init). [...] > index c76122f22294..928e79c7549c 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -886,6 +886,15 @@ static inline void prep_compound_tail(struct page *tail, > set_page_private(tail, 0); > } > > +static inline void init_compound_tail(struct page *tail, > + const struct page *head, unsigned int order, struct zone *zone) > +{ In light of the refcount rework (lock/frozen bit), it might make sense to directly set the page count here to 0, so it's easier to spot and consider later. set_page_count(tail, 0) > + atomic_set(&tail->_mapcount, -1); > + set_page_node(tail, zone_to_nid(zone)); > + set_page_zone(tail, zone_idx(zone)); > + prep_compound_tail(tail, head, order); > +} > + Nothing else jumped at me Acked-by: David Hildenbrand (Arm) -- Cheers, David