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Mon, 2 Feb 2026 10:56:43 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , 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, Kiryl Shutsemau Subject: [PATCHv6 00/17] mm: Eliminate fake head pages from vmemmap optimization Date: Mon, 2 Feb 2026 15:56:16 +0000 Message-ID: <20260202155634.650837-1-kas@kernel.org> X-Mailer: git-send-email 2.51.2 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: E55044000E X-Stat-Signature: qpwy6bkajixn7xab5jzmusexrh6tbut4 X-Rspam-User: X-HE-Tag: 1770047807-166418 X-HE-Meta: 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 7SH+jiqt n0lv/vRLFVyGtkziTGtEEdUu0TN2QmFAaC/Sn/ITXSn3xSUgkziGYEfr8ciwO7aX5iMJup2kfVRiC4p1ZH/GJ1omJK4iiPDUu/6aAUh1ShsO6lVpAzkRE8t1mfck7MAnZ2jWWC8rDQlxxTi601941vO7RCvGjAiDggfqP02rKfVi5ExerziPEmSUftCw6Lb/qRV9wneiN1dATf7alVuSP1JmNLwOeXbNIFHp9O67XYvkq+XU= 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: This series removes "fake head pages" from the HugeTLB vmemmap optimization (HVO) by changing how tail pages encode their relationship to the head page. It simplifies compound_head() and page_ref_add_unless(). Both are in the hot path. Background ========== HVO reduces memory overhead by freeing vmemmap pages for HugeTLB pages and remapping the freed virtual addresses to a single physical page. Previously, all tail page vmemmap entries were remapped to the first vmemmap page (containing the head struct page), creating "fake heads" - tail pages that appear to have PG_head set when accessed through the deduplicated vmemmap. This required special handling in compound_head() to detect and work around fake heads, adding complexity and overhead to a very hot path. New Approach ============ For architectures/configs where sizeof(struct page) is a power of 2 (the common case), this series changes how position of the head page is encoded in the tail pages. Instead of storing a pointer to the head page, the ->compound_info (renamed from ->compound_head) now stores a mask. The mask can be applied to any tail page's virtual address to compute the head page address. Critically, all tail pages of the same order now have identical compound_info values, regardless of which compound page they belong to. The key insight is that all tail pages of the same order now have identical compound_info values, regardless of which compound page they belong to. This allows a single page of tail struct pages to be shared across all huge pages of the same order on a NUMA node. Benefits ======== 1. Simplified compound_head(): No fake head detection needed, can be implemented in a branchless manner. 2. Simplified page_ref_add_unless(): RCU protection removed since there's no race with fake head remapping. 3. Cleaner architecture: The shared tail pages are truly read-only and contain valid tail page metadata. If sizeof(struct page) is not power-of-2, there are no functional changes. HVO is not supported in this configuration. I had hoped to see performance improvement, but my testing thus far has shown either no change or only a slight improvement within the noise. Series Organization =================== Patch 1: Preparation - move MAX_FOLIO_ORDER to mmzone.h Patches 2-4: Refactoring - interface changes, field rename, code movement Patches 5-6: Arch fixes - align vmemmap for riscv and LoongArch Patch 7: Core change - new mask-based compound_head() encoding Patch 8: Correctness fix - page_zonenum() must use head page Patch 9: Add memmap alignment check for compound_info_has_mask() Patch 10: Refactor vmemmap_walk for new design Patch 11: Eliminate fake heads with shared tail pages Patches 12-15: Cleanup - remove fake head infrastructure Patch 16: Documentation update Patch 17: Get rid of opencoded compound_head() in page_slab() Changes in v6: ============== - Simplify memmap alignment check in mm/sparse.c: use VM_BUG_ON() (Muchun) - Store struct page pointers in vmemmap_tails[] instead of PFNs. (Muchun) - Fix build error on powerpc due to negative NR_VMEMMAP_TAILS. Changes in v5: ============== - Rebased to mm-everything-2026-01-27-04-35 - Add arch-specific patches to align vmemmap to maximal folio size for riscv and LoongArch architectures. - Strengthen the memmap alignment check in mm/sparse.c: use BUG() for CONFIG_DEBUG_VM, WARN() otherwise. (Muchun) - Use cmpxchg() instead of hugetlb_lock to update vmemmap_tails array. (Muchun) - Update page_slab(). Changes in v4: ============== - Fix build issues due to linux/mmzone.h <-> linux/pgtable.h dependency loop by avoiding including linux/pgtable.h into linux/mmzone.h - Rework vmemmap_remap_alloc() interface. (Muchun) - Use &folio->page instead of folio address for optimization target. (Muchun) Changes in v3: ============== - Fixed error recovery path in vmemmap_remap_free() to pass correct start address for TLB flush. (Muchun) - Wrapped the mask-based compound_info encoding within CONFIG_SPARSEMEM_VMEMMAP check via compound_info_has_mask(). For other memory models, alignment guarantees are harder to verify. (Muchun) - Updated vmemmap_dedup.rst documentation wording: changed "vmemmap_tail shared for the struct hstate" to "A single, per-node page frame shared among all hugepages of the same size". (Muchun) - Fixed build error with MAX_FOLIO_ORDER expanding to undefined PUD_ORDER in certain configurations. (kernel test robot) Changes in v2: ============== - Handle boot-allocated huge pages correctly. (Frank) - Changed from per-hstate vmemmap_tail to per-node vmemmap_tails[] array in pglist_data. (Muchun) - Added spin_lock(&hugetlb_lock) protection in vmemmap_get_tail() to fix a race condition where two threads could both allocate tail pages. The losing thread now properly frees its allocated page. (Usama) - Add warning if memmap is not aligned to MAX_FOLIO_SIZE, which is required for the mask approach. (Muchun) - Make page_zonenum() use head page - correctness fix since shared tail pages cannot have valid zone information. (Muchun) - Added 'const' qualifier to head parameter in set_compound_head() and prep_compound_tail(). (Usama) - Updated commit messages. Kiryl Shutsemau (17): mm: Move MAX_FOLIO_ORDER definition to mmzone.h mm: Change the interface of prep_compound_tail() mm: Rename the 'compound_head' field in the 'struct page' to 'compound_info' mm: Move set/clear_compound_head() next to compound_head() riscv/mm: Align vmemmap to maximal folio size LoongArch/mm: Align vmemmap to maximal folio size mm: Rework compound_head() for power-of-2 sizeof(struct page) mm: Make page_zonenum() use head page mm/sparse: Check memmap alignment for compound_info_has_mask() mm/hugetlb: Refactor code around vmemmap_walk mm/hugetlb: Remove fake head pages mm: Drop fake head checks hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU mm/hugetlb: Remove hugetlb_optimize_vmemmap_key static key mm: Remove the branch from compound_head() hugetlb: Update vmemmap_dedup.rst mm/slab: Use compound_head() in page_slab() .../admin-guide/kdump/vmcoreinfo.rst | 2 +- Documentation/mm/vmemmap_dedup.rst | 62 ++-- arch/loongarch/include/asm/pgtable.h | 3 +- arch/riscv/mm/init.c | 3 +- include/linux/mm.h | 31 -- include/linux/mm_types.h | 20 +- include/linux/mmzone.h | 47 +++ include/linux/page-flags.h | 167 +++++----- include/linux/page_ref.h | 8 +- include/linux/types.h | 2 +- kernel/vmcore_info.c | 2 +- mm/hugetlb.c | 8 +- mm/hugetlb_vmemmap.c | 288 ++++++++---------- mm/internal.h | 12 +- mm/mm_init.c | 2 +- mm/page_alloc.c | 4 +- mm/slab.h | 8 +- mm/sparse-vmemmap.c | 43 ++- mm/sparse.c | 7 + mm/util.c | 16 +- 20 files changed, 363 insertions(+), 372 deletions(-) -- 2.51.2