Re: [External] Re: [PATCH] mm: hugetlb: fix missing put_page in gather_surplus_pages()

[Muchun Song]

On Tue, Jan 26, 2021 at 12:31 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 1/25/21 7:10 PM, Muchun Song wrote:
> > The VM_BUG_ON_PAGE avoids the generation of any code, even if that
> > expression has side-effects when !CONFIG_DEBUG_VM.
> >
> > Fixes: e5dfacebe4a4 ("mm/hugetlb.c: just use put_page_testzero() instead of page_count()")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > ---
> >  mm/hugetlb.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
>
> Thanks for finding and fixing this!  My bad for not noticing when the bug
> was introduced.
>
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index a6bad1f686c5..082ed643020b 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2047,13 +2047,16 @@ static int gather_surplus_pages(struct hstate *h, long delta)
> >
> >       /* Free the needed pages to the hugetlb pool */
> >       list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
> > +             int zeroed;
> > +
> >               if ((--needed) < 0)
> >                       break;
> >               /*
> >                * This page is now managed by the hugetlb allocator and has
> >                * no users -- drop the buddy allocator's reference.
> >                */
> > -             VM_BUG_ON_PAGE(!put_page_testzero(page), page);
> > +             zeroed = put_page_testzero(page);
> > +             VM_BUG_ON_PAGE(!zeroed, page);
> >               enqueue_huge_page(h, page);
>
> I was wondering why this was not causing any problems.  We are putting the
> hugetlb page on the free list with a count of 1.  There is no check in the
> enqueue code.  When we dequeue the page, set_page_refcounted() is used to
> set the count to 1 without looking at the current value.  And, all the other
> VM_DEBUG macros are off so we mostly do not notice the bug.

Yeah. You are right. I also thought about this question.
It is a very hidden bug.

>
> Thanks again,
>
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>
> --
> Mike Kravetz
>
> >       }
> >  free:
> >