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Wed, 21 Jan 2026 11:22:55 -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 , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 00/14] mm: Eliminate fake head pages from vmemmap optimization Date: Wed, 21 Jan 2026 16:22:37 +0000 Message-ID: <20260121162253.2216580-1-kas@kernel.org> X-Mailer: git-send-email 2.51.2 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Stat-Signature: 4rwccgpkkkqbkkbymxzkcnq18k7b38qn X-Rspamd-Queue-Id: C09CC1C000E X-HE-Tag: 1769012578-849782 X-HE-Meta: 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 oR6zxIl9 kWS+lFto1bGrktw3ElEybh6FoQvGVEDDPAs3+terfGWgAGD10WG4fBDWzziIrDPTJlMheypgsJRAcywLvsknNyTCcdAhKaxkqNknzMn3NSs8Xuhn7eVZQC6beFiccTSvCc5zhR3byUR2kK1qDXgRnU2LJqUEPh+RGB31dmrkL3yh9tfKO5xYvDSGMzxC/7orPTwIUW3GJNvqrvO5z9Tcq1U7LKJDds4gemJGCgkUmDOeoIRU= 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 Patch 5: Core change - new mask-based compound_head() encoding Patch 6: Correctness fix - page_zonenum() must use head page Patch 7: Add memmap alignment check for compound_info_has_mask() Patch 8: Refactor vmemmap_walk for new design Patch 9: Eliminate fake heads with shared tail pages Patches 10-13: Cleanup - remove fake head infrastructure Patch 14: Documentation update 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 (14): 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() 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 .../admin-guide/kdump/vmcoreinfo.rst | 2 +- Documentation/mm/vmemmap_dedup.rst | 62 ++-- 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 | 300 ++++++++---------- mm/internal.h | 12 +- mm/mm_init.c | 2 +- mm/page_alloc.c | 4 +- mm/slab.h | 2 +- mm/sparse-vmemmap.c | 44 ++- mm/sparse.c | 5 + mm/util.c | 16 +- 18 files changed, 369 insertions(+), 365 deletions(-) -- 2.51.2