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    SubjectRe: [PATCH v2 1/2] khugepaged: enable collapse pmd for pte-mapped THP
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    > On Aug 1, 2019, at 7:50 AM, Oleg Nesterov <oleg@redhat.com> wrote:
    >
    > On 07/31, Song Liu wrote:
    >>
    >> +static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
    >> + unsigned long addr)
    >> +{
    >> + struct mm_slot *mm_slot;
    >> + int ret = 0;
    >> +
    >> + /* hold mmap_sem for khugepaged_test_exit() */
    >> + VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
    >> + VM_BUG_ON(addr & ~HPAGE_PMD_MASK);
    >> +
    >> + if (unlikely(khugepaged_test_exit(mm)))
    >> + return 0;
    >> +
    >> + if (!test_bit(MMF_VM_HUGEPAGE, &mm->flags) &&
    >> + !test_bit(MMF_DISABLE_THP, &mm->flags)) {
    >> + ret = __khugepaged_enter(mm);
    >> + if (ret)
    >> + return ret;
    >> + }
    >
    > could you explain why do we need mm->mmap_sem, khugepaged_test_exit() check
    > and __khugepaged_enter() ?

    If the mm doesn't have a mm_slot, we would like to create one here (by
    calling __khugepaged_enter()).

    This happens when the THP is created by another mm, or by tmpfs with
    "huge=always"; and then page table of this mm got split by split_huge_pmd().
    With current kernel, this happens when we attach/detach uprobe to a file
    in tmpfs with huge=always.

    Does this answer your question?

    Thanks,
    Song

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