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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/30/25 7:56 AM, Jann Horn wrote: > On Fri, Apr 4, 2025 at 4:18 AM Anthony Yznaga wrote: >> In preparation for enabling the handling of page faults in an mshare >> region provide a way to link an mshare shared page table to a process >> page table and otherwise find the actual vma in order to handle a page >> fault. Modify the unmap path to ensure that page tables in mshare regions >> are unlinked and kept intact when a process exits or an mshare region >> is explicitly unmapped. >> >> Signed-off-by: Khalid Aziz >> Signed-off-by: Matthew Wilcox (Oracle) >> Signed-off-by: Anthony Yznaga > [...] >> diff --git a/mm/memory.c b/mm/memory.c >> index db558fe43088..68422b606819 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c > [...] >> @@ -259,7 +260,10 @@ static inline void free_p4d_range(struct mmu_gather *tlb, pgd_t *pgd, >> next = p4d_addr_end(addr, end); >> if (p4d_none_or_clear_bad(p4d)) >> continue; >> - free_pud_range(tlb, p4d, addr, next, floor, ceiling); >> + if (unlikely(shared_pud)) >> + p4d_clear(p4d); >> + else >> + free_pud_range(tlb, p4d, addr, next, floor, ceiling); >> } while (p4d++, addr = next, addr != end); >> >> start &= PGDIR_MASK; > [...] >> +static void mshare_vm_op_unmap_page_range(struct mmu_gather *tlb, >> + struct vm_area_struct *vma, >> + unsigned long addr, unsigned long end, >> + struct zap_details *details) >> +{ >> + /* >> + * The msharefs vma is being unmapped. Do not unmap pages in the >> + * mshare region itself. >> + */ >> +} > > Unmapping a VMA has three major phases: > > 1. unlinking the VMA from the VMA tree > 2. removing the VMA contents > 3. removing unneeded page tables > > The MM subsystem broadly assumes that after phase 2, no stuff is > mapped in the region anymore and therefore changes to the backing file > don't need to TLB-flush this VMA anymore, and unlinks the mapping from > rmaps and such. If munmap() of an mshare region only removes the > mapping of shared page tables in step 3, as implemented here, that > means things like TLB flushes won't be able to discover all > currently-existing mshare mappings of a host MM through rmap walks. > > I think it would make more sense to remove the links to shared page > tables in step 2 (meaning in mshare_vm_op_unmap_page_range), just like > hugetlb does, and not modify free_pgtables(). That makes sense. I'll make this change. Thanks! > >> static const struct vm_operations_struct msharefs_vm_ops = { >> .may_split = mshare_vm_op_split, >> .mprotect = mshare_vm_op_mprotect, >> + .unmap_page_range = mshare_vm_op_unmap_page_range, >> }; >> >> /* >> -- >> 2.43.5 >>