Re: [PATCH] mm/damon: Validate if the pmd entry is present before accessing

[Baolin Wang]

在 8/18/2022 10:41 AM, Muchun Song 写道:
> > On Aug 17, 2022, at 14:21, Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> >
> > The pmd_huge() is used to validate if the pmd entry is mapped by a huge
> > page, also including the case of non-present (migration or hwpoisoned)
> > pmd entry on arm64 or x86 architectures. Thus we should validate if it
> > is present before making the pmd entry old or getting young state,
> > otherwise we can not get the correct corresponding page.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> > mm/damon/vaddr.c | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> > index 3c7b9d6..1d16c6c 100644
> > --- a/mm/damon/vaddr.c
> > +++ b/mm/damon/vaddr.c
> > @@ -304,6 +304,11 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
> >
> > 	if (pmd_huge(*pmd)) {
> > 		ptl = pmd_lock(walk->mm, pmd);
> > +		if (!pmd_present(*pmd)) {
>
> Unluckily, we should use pte_present here. See commit c9d398fa23788. We can use
> huge_ptep_get() to get a hugetlb pte, so it’s better to put the check after
> pmd_huge.

IMO this is not the case for hugetlb, and the hugetlb case will be 
handled by damon_mkold_hugetlb_entry(), which already used pte_present() 
for hugetlb case.

> Cc Mike to make sure I am not missing something.
>
> Muchun,
> Thanks.
>
> > +			spin_unlock(ptl);
> > +			return 0;
> > +		}
> > +
> > 		if (pmd_huge(*pmd)) {
> > 			damon_pmdp_mkold(pmd, walk->mm, addr);
> > 			spin_unlock(ptl);
> > @@ -431,6 +436,11 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > 	if (pmd_huge(*pmd)) {
> > 		ptl = pmd_lock(walk->mm, pmd);
> > +		if (!pmd_present(*pmd)) {
> > +			spin_unlock(ptl);
> > +			return 0;
> > +		}
> > +
> > 		if (!pmd_huge(*pmd)) {
> > 			spin_unlock(ptl);
> > 			goto regular_page;
> > --
> > 1.8.3.1