Re: [PATCH 25/31] mm/gup: remove FOLL_SPLIT_PMD use of pmd_trans_unstable()

[Yang Shi]

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
>