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

[Yang Shi <shy828301@gmail.com>]

On Mon, May 22, 2023 at 7:26 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Sun, May 21, 2023 at 10:22 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.
> > (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>
> > ---
> >  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.

Oops, the above comment is not accurate. It will call
follow_page_pte() instead of returning -EBUSY if pmd is none. For
other unstable cases, it will return -ENOMEM instead of -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
> >