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List-Unsubscribe: On Fri, Dec 12, 2025 at 08:10:19AM +0100, David Hildenbrand (Red Hat) wrote: > As reported, ever since commit 1013af4f585f ("mm/hugetlb: fix > huge_pmd_unshare() vs GUP-fast race") we can end up in some situations > where we perform so many IPI broadcasts when unsharing hugetlb PMD page > tables that it severely regresses some workloads. > > In particular, when we fork()+exit(), or when we munmap() a large > area backed by many shared PMD tables, we perform one IPI broadcast per > unshared PMD table. > [...snip...] > Fixes: 1013af4f585f ("mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race") > Reported-by: Uschakow, Stanislav" > Closes: https://lore.kernel.org/all/4d3878531c76479d9f8ca9789dc6485d@amazon.de/ > Tested-by: Laurence Oberman > Cc: > Signed-off-by: David Hildenbrand (Red Hat) > --- > include/asm-generic/tlb.h | 74 ++++++++++++++++++++++- > include/linux/hugetlb.h | 19 +++--- > mm/hugetlb.c | 121 ++++++++++++++++++++++---------------- > mm/mmu_gather.c | 7 +++ > mm/mprotect.c | 2 +- > mm/rmap.c | 25 +++++--- > 6 files changed, 179 insertions(+), 69 deletions(-) > > @@ -6522,22 +6511,16 @@ long hugetlb_change_protection(struct vm_area_struct *vma, > pte = huge_pte_clear_uffd_wp(pte); > huge_ptep_modify_prot_commit(vma, address, ptep, old_pte, pte); > pages++; > + tlb_remove_huge_tlb_entry(h, tlb, ptep, address); > } > > next: > spin_unlock(ptl); > cond_resched(); > } > - /* > - * There is nothing protecting a previously-shared page table that we > - * unshared through huge_pmd_unshare() from getting freed after we > - * release i_mmap_rwsem, so flush the TLB now. If huge_pmd_unshare() > - * succeeded, flush the range corresponding to the pud. > - */ > - if (shared_pmd) > - flush_hugetlb_tlb_range(vma, range.start, range.end); > - else > - flush_hugetlb_tlb_range(vma, start, end); > + > + tlb_flush_mmu_tlbonly(tlb); > + huge_pmd_unshare_flush(tlb, vma); Shouldn't we teach mmu_gather that it has to call flush_hugetlb_tlb_range() instead of ordinary TLB flush routine, otherwise it will break ARCHes that has "special requirements" for evicting hugetlb backing TLB entries? > /* > * No need to call mmu_notifier_arch_invalidate_secondary_tlbs() we are > * downgrading page table protection not changing it to point to a new -- Cheers, Harry / Hyeonggon