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    SubjectRe: [PATCH v2 25/32] mm/gup: remove FOLL_SPLIT_PMD use of pmd_trans_unstable()
    On Thu, Jun 8, 2023 at 6:40 PM Hugh Dickins <hughd@google.com> wrote:
    >
    > There is now no reason for follow_pmd_mask()'s FOLL_SPLIT_PMD block to
    > distinguish huge_zero_page from a normal THP: follow_page_pte() handles
    > any instability, and here it's a good idea to replace any pmd_none(*pmd)
    > by a page table a.s.a.p, in the huge_zero_page case as for a normal THP;
    > and this removes an unnecessary possibility of -EBUSY failure.
    >
    > (Hmm, couldn't the normal THP case have hit an unstably refaulted THP
    > before? But there are only two, exceptional, users of FOLL_SPLIT_PMD.)
    >
    > Signed-off-by: Hugh Dickins <hughd@google.com>

    Reviewed-by: Yang Shi <shy828301@gmail.com>

    > ---
    > mm/gup.c | 19 ++++---------------
    > 1 file changed, 4 insertions(+), 15 deletions(-)
    >
    > diff --git a/mm/gup.c b/mm/gup.c
    > index bb67193c5460..4ad50a59897f 100644
    > --- a/mm/gup.c
    > +++ b/mm/gup.c
    > @@ -681,21 +681,10 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
    > return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
    > }
    > if (flags & FOLL_SPLIT_PMD) {
    > - int ret;
    > - page = pmd_page(*pmd);
    > - if (is_huge_zero_page(page)) {
    > - spin_unlock(ptl);
    > - ret = 0;
    > - split_huge_pmd(vma, pmd, address);
    > - if (pmd_trans_unstable(pmd))
    > - ret = -EBUSY;
    > - } else {
    > - spin_unlock(ptl);
    > - split_huge_pmd(vma, pmd, address);
    > - ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
    > - }
    > -
    > - return ret ? ERR_PTR(ret) :
    > + spin_unlock(ptl);
    > + split_huge_pmd(vma, pmd, address);
    > + /* If pmd was left empty, stuff a page table in there quickly */
    > + return pte_alloc(mm, pmd) ? ERR_PTR(-ENOMEM) :
    > follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
    > }
    > page = follow_trans_huge_pmd(vma, address, pmd, flags);
    > --
    > 2.35.3
    >

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