Re: [PATCH mm-unstable] Revert "mm/khugepaged: remove redundant transhuge_vma_suitable() check"

["Zach O'Keefe" <zokeefe@google.com>]

On Wed, Jul 20, 2022 at 10:22 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Wed, Jul 20, 2022 at 4:13 AM Zach O'Keefe <zokeefe@google.com> wrote:
> >
> > A pmd should not cross a VMA boundary, which is normally enforced by
> > vma_adjust_trans_huge(), and assumed by e.g. __split_huge_pmd_locked().
> >
> > In this regard, the transhuge_vma_suitable() check in
> > hugepage_vma_check() is not redundant with the transhuge_vma_suitable()
> > check previously in hugepage_vma_revalidate().
> >
> > The former validates the VMA itself, and checks that *some* memory
> > in the VMA is suitable to collapse while the latter validates if
> > collapsing at a specific address is suitable.  By removing the check on
> > the faulting address, we've inadvertently allowed collapse of a pmd that
> > can cross vma->vm_end. Revert this change.
>
> Aha, yeah, nice catch.
>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
>

Thanks Yang. Also, hughd found it :) In hindsight, I think it's
actually customary to add a "Reported-by: Hugh Dickins
<hughd@google.com>" - but since the previous patch will just be
dropped and never see the light of day, I guess the value there is
diminished. Anyways - credit goes to Hugh :)

Thanks,
Zach

> >
> > Fixes: 143776e7512e ("mm/khugepaged: remove redundant transhuge_vma_suitable() check")
> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > ---
> > Apologies, Andrew. I think you've put the series description into this
> > first patch (thank you).  Do you mind moving it into the next patch in the
> > series,
> > "mm: khugepaged: don't carry huge page to the next loop for !CONFIG_NUMA"?
> > Note that the "mm: userspace hugepage collapse, v7" series doesn't actually
> > depend on this patch, it was just a cleanup (and thus perhaps I shouldn't have
> > included it in the series in the first place).
> > ---
> >  mm/khugepaged.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 2db6d0dd2981..69990dacde14 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -855,6 +855,8 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> >         if (!vma)
> >                 return SCAN_VMA_NULL;
> >
> > +       if (!transhuge_vma_suitable(vma, address))
> > +               return SCAN_ADDRESS_RANGE;
> >         if (!hugepage_vma_check(vma, vma->vm_flags, false, false,
> >                                 cc->is_khugepaged))
> >                 return SCAN_VMA_CHECK;
> > --
> > 2.37.0.170.g444d1eabd0-goog
> >