On Sun, May 21, 2023 at 10:22 PM Hugh Dickins <hughd@xxxxxxxxxx> 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.
> (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@xxxxxxxxxx>
> ---
> 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;
IIUC the pmd_trans_unstable() check was transferred to the implicit
pmd_none() in pte_alloc(). But it will return -ENOMEM instead of
-EBUSY. Won't it break some userspace? Or the pmd_trans_unstable() is
never true? If so it seems worth mentioning in the commit log about
this return value change.
> - } 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
>
