Re: [PATCH v3 4/6] mm/hugetlb: add migration entry check in hugetlb_change_protection

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Wed, Sep 03, 2014 at 06:06:34PM -0700, Hugh Dickins wrote:
> On Thu, 28 Aug 2014, Naoya Horiguchi wrote:
> 
> > There is a race condition between hugepage migration and change_protection(),
> > where hugetlb_change_protection() doesn't care about migration entries and
> > wrongly overwrites them. That causes unexpected results like kernel crash.
> > 
> > This patch adds is_hugetlb_entry_(migration|hwpoisoned) check in this
> > function to do proper actions.
> > 
> > ChangeLog v3:
> > - handle migration entry correctly (instead of just skipping)
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Cc: <stable@vger.kernel.org> # [2.6.36+]
> 
> 2.6.36+?  For the hwpoisoned part of it, I suppose.
> Then you'd better mentioned the hwpoisoned case in the comment above.

OK, I'll update the description and the subject.

> > ---
> >  mm/hugetlb.c | 21 ++++++++++++++++++++-
> >  1 file changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git mmotm-2014-08-25-16-52.orig/mm/hugetlb.c mmotm-2014-08-25-16-52/mm/hugetlb.c
> > index 2aafe073cb06..1ed9df6def54 100644
> > --- mmotm-2014-08-25-16-52.orig/mm/hugetlb.c
> > +++ mmotm-2014-08-25-16-52/mm/hugetlb.c
> > @@ -3362,7 +3362,26 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> >  			spin_unlock(ptl);
> >  			continue;
> >  		}
> > -		if (!huge_pte_none(huge_ptep_get(ptep))) {
> > +		pte = huge_ptep_get(ptep);
> > +		if (unlikely(is_hugetlb_entry_hwpoisoned(pte))) {
> > +			spin_unlock(ptl);
> > +			continue;
> > +		}
> > +		if (unlikely(is_hugetlb_entry_migration(pte))) {
> > +			swp_entry_t entry = pte_to_swp_entry(pte);
> > +
> > +			if (is_write_migration_entry(entry)) {
> > +				pte_t newpte;
> > +
> > +				make_migration_entry_read(&entry);
> > +				newpte = swp_entry_to_pte(entry);
> > +				set_pte_at(mm, address, ptep, newpte);
> 
> set_huge_pte_at.

Fixed, thanks.

> 
> (As usual, I can't bear to see these is_hugetlb_entry_hwpoisoned and
> is_hugetlb_entry_migration examples go past without bleating about
> wanting to streamline them a little; but agreed last time to leave
> that to some later cleanup once all the stable backports are stable.)

Yes, these two check routines need cleanup.
I'll do it in separate work later.

> > +				pages++;
> > +			}
> > +			spin_unlock(ptl);
> > +			continue;
> > +		}
> > +		if (!huge_pte_none(pte)) {
> >  			pte = huge_ptep_get_and_clear(mm, address, ptep);
> >  			pte = pte_mkhuge(huge_pte_modify(pte, newprot));
> >  			pte = arch_make_huge_pte(pte, vma, NULL, 0);
> > -- 
> > 1.9.3
> 

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