Re: [External] Re: [PATCH v2 6/6] mm: hugetlb: remove VM_BUG_ON_PAGE from page_huge_active

[Muchun Song <songmuchun@bytedance.com>]

On Sat, Jan 9, 2021 at 6:24 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 1/6/21 12:47 AM, Muchun Song wrote:
> > The page_huge_active() can be called from scan_movable_pages() which
> > do not hold a reference count to the HugeTLB page. So when we call
> > page_huge_active() from scan_movable_pages(), the HugeTLB page can
> > be freed parallel. Then we will trigger a BUG_ON which is in the
> > page_huge_active() when CONFIG_DEBUG_VM is enabled. Just remove the
> > VM_BUG_ON_PAGE.
> >
> > Fixes: 7e1f049efb86 ("mm: hugetlb: cleanup using paeg_huge_active()")
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> > ---
> >  mm/hugetlb.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 67200dd25b1d..7a24ed28ec4f 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1372,7 +1372,6 @@ struct hstate *size_to_hstate(unsigned long size)
> >   */
> >  bool page_huge_active(struct page *page)
> >  {
> > -     VM_BUG_ON_PAGE(!PageHuge(page), page);
> >       return PageHead(page) && PagePrivate(&page[1]);
> >  }
>
> After more thought, should that return statement be changed to?
>         return PageHeadHuge(page) && PagePrivate(&page[1]);

Agree.

>
> We only want to test that PagePrivate flag for hugetlb head pages.
> Although, the possibility that the hugetlb page was freed and turned
> into another compound page in this race window is REALLY small.

Yeah. Thanks. I will update to PageHeadHuge().

> --
> Mike Kravetz