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

[Hugh Dickins]

On Mon, 22 May 2023, Yang Shi wrote:
> On Mon, May 22, 2023 at 7:26 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
> > 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.

Thanks a lot for looking at these, but I disagree here.

> 
> Oops, the above comment is not accurate. It will call
> follow_page_pte() instead of returning -EBUSY if pmd is none.

Yes.  Ignoring secondary races, if pmd is none, pte_alloc() will allocate
an empty page table there, follow_page_pte() find !pte_present and return
NULL; or if pmd is not none, follow_page_pte() will return no_page_table()
i.e. NULL.  And page NULL ends up with __get_user_pages() having another
go round, instead of failing with -EBUSY.

Which I'd say is better handling for such a transient case - remember,
it's split_huge_pmd() (which should always succeed, but might be raced)
in use there, not split_huge_page() (which might take years for pins to
be removed before it can succeed).

> For other unstable cases, it will return -ENOMEM instead of -EBUSY.

I don't think so: the possibly-failing __pte_alloc() only gets called
in the pmd_none() case.

Hugh

> 
> >
> > > -               } 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