Hugetlb currently expands MMU notifier ranges to PUD boundaries whenever PMD sharing is possible. That is only needed when huge_pmd_unshare() actually detaches a shared PMD page table, because clearing the PUD invalidates the whole PUD-sized virtual address range. For hugetlbfs hole punch, and similarly for other hugetlb unmap paths, a shared mapping can pass the "PMD sharing is possible" range test in adjust_range_if_pmd_sharing_possible() even when the hugetlbfs file does not currently have any shared PMD page tables. KVM then receives a 1G invalidation for a 2M operation and zaps unrelated secondary mappings, so the guest has to fault them back in. Avoid this by tracking active PMD-sharing attachments per hugetlbfs inode. The count is incremented only after huge_pmd_share() successfully installs a shared PMD table, and decremented when __huge_pmd_unshare() actually detaches one. Since huge_pmd_share() can run concurrently under i_mmap_lock_read(), use atomic operations for the count. A zero count is used to skip the conservative notifier range expansion only after excluding concurrent PMD sharing with the mapping write lock. On a Redis-in-VM workload that punches cold 2M hugetlb pages, this patch improves P99 QPS stability while punching pages, reducing the QPS degradation ratio from 7.09% to 1.45%. Reported-by: aiqi.i7 Signed-off-by: Li Zhe --- v2: https://lore.kernel.org/all/20260922090749.24905-1-lizhe.67@bytedance.com/ v1: https://lore.kernel.org/all/20260831091023.66581-1-lizhe.67@bytedance.com/ ChangeLogs: v2->v3: - Rework the sticky state based on Andrew's feedback: use a per-inode counter of active PMD-sharing attachments, so files can return to the no-active-sharing state after PMD sharing ends. v1->v2: - Rework the implementation based on David's suggestion: remember whether PMD sharing ever happened for a hugetlbfs file, and skip the conservative notifier range expansion while it has not. This avoids the per-unmap page-table walk. fs/hugetlbfs/inode.c | 1 + include/linux/hugetlb.h | 43 +++++++++++++++++++++++++++++++++++++++++ mm/hugetlb.c | 13 ++++++++++--- 3 files changed, 54 insertions(+), 3 deletions(-) diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c index 7611a8470ea26..78e27ce0a6f63 100644 --- a/fs/hugetlbfs/inode.c +++ b/fs/hugetlbfs/inode.c @@ -921,6 +921,7 @@ static struct inode *hugetlbfs_get_inode(struct super_block *sb, simple_inode_init_ts(inode); info->resv_map = resv_map; info->seals = F_SEAL_SEAL; + hugetlbfs_pmd_sharing_init(inode); switch (mode & S_IFMT) { default: init_special_inode(inode, mode, dev); diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h index 16c4c4caa126c..6b4f92b7f7ae4 100644 --- a/include/linux/hugetlb.h +++ b/include/linux/hugetlb.h @@ -12,6 +12,7 @@ #include #include #include +#include #include #include #include @@ -509,6 +510,9 @@ struct hugetlbfs_inode_info { struct inode vfs_inode; struct resv_map *resv_map; unsigned int seals; +#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING + atomic_t pmd_sharing_count; +#endif }; static inline struct hugetlbfs_inode_info *HUGETLBFS_I(struct inode *inode) @@ -516,6 +520,45 @@ static inline struct hugetlbfs_inode_info *HUGETLBFS_I(struct inode *inode) return container_of(inode, struct hugetlbfs_inode_info, vfs_inode); } +#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING +static inline void hugetlbfs_pmd_sharing_init(struct inode *inode) +{ + atomic_set(&HUGETLBFS_I(inode)->pmd_sharing_count, 0); +} + +static inline void hugetlbfs_pmd_sharing_inc(struct inode *inode) +{ + atomic_inc(&HUGETLBFS_I(inode)->pmd_sharing_count); +} + +static inline void hugetlbfs_pmd_sharing_dec(struct inode *inode) +{ + atomic_dec(&HUGETLBFS_I(inode)->pmd_sharing_count); +} + +/* + * A 32-bit counter can theoretically wrap, but doing so would require + * billions of active PMD-sharing attachments to the same inode and is not + * expected in practice. Treat any non-zero value as active so a wrapped + * negative value still takes the conservative notifier range. + */ +static inline bool hugetlbfs_pmd_sharing_active(struct inode *inode) +{ + return atomic_read(&HUGETLBFS_I(inode)->pmd_sharing_count) != 0; +} +#else +static inline void hugetlbfs_pmd_sharing_init(struct inode *inode) {} + +static inline void hugetlbfs_pmd_sharing_inc(struct inode *inode) {} + +static inline void hugetlbfs_pmd_sharing_dec(struct inode *inode) {} + +static inline bool hugetlbfs_pmd_sharing_active(struct inode *inode) +{ + return false; +} +#endif + extern const struct vm_operations_struct hugetlb_vm_ops; struct file *hugetlb_file_setup(const char *name, size_t size, vma_flags_t acct, int creat_flags, int page_size_log); diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 4f6f58bf3db6c..cb27c06d1f7a7 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5361,10 +5361,12 @@ void __hugetlb_zap_begin(struct vm_area_struct *vma, if (!vma->vm_file) /* hugetlbfs_file_mmap error */ return; - adjust_range_if_pmd_sharing_possible(vma, start, end); hugetlb_vma_lock_write(vma); - if (vma->vm_file) + if (vma->vm_file) { i_mmap_lock_write(vma->vm_file->f_mapping); + if (hugetlbfs_pmd_sharing_active(file_inode(vma->vm_file))) + adjust_range_if_pmd_sharing_possible(vma, start, end); + } } void __hugetlb_zap_end(struct vm_area_struct *vma, @@ -5403,7 +5405,10 @@ void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start, mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, start, end); - adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end); + i_mmap_assert_write_locked(vma->vm_file->f_mapping); + if (hugetlbfs_pmd_sharing_active(file_inode(vma->vm_file))) + adjust_range_if_pmd_sharing_possible(vma, &range.start, + &range.end); mmu_notifier_invalidate_range_start(&range); tlb_gather_mmu(&tlb, vma->vm_mm); @@ -7006,6 +7011,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma, if (pud_none(*pud)) { pud_populate(mm, pud, (pmd_t *)((unsigned long)spte & PAGE_MASK)); + hugetlbfs_pmd_sharing_inc(file_inode(vma->vm_file)); mm_inc_nr_pmds(mm); } else { ptdesc_pmd_pts_dec(virt_to_ptdesc(spte)); @@ -7037,6 +7043,7 @@ static int __huge_pmd_unshare(struct mmu_gather *tlb, pud_clear(pud); tlb_unshare_pmd_ptdesc(tlb, virt_to_ptdesc(ptep), addr); + hugetlbfs_pmd_sharing_dec(file_inode(vma->vm_file)); mm_dec_nr_pmds(mm); return 1; -- 2.20.1