Re: [External] Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Jan 7, 2021 at 12:56 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 06-01-21 16:47:36, Muchun Song wrote:
> > There is a race condition between __free_huge_page()
> > and dissolve_free_huge_page().
> >
> > CPU0:                         CPU1:
> >
> > // page_count(page) == 1
> > put_page(page)
> >   __free_huge_page(page)
> >                               dissolve_free_huge_page(page)
> >                                 spin_lock(&hugetlb_lock)
> >                                 // PageHuge(page) && !page_count(page)
> >                                 update_and_free_page(page)
> >                                 // page is freed to the buddy
> >                                 spin_unlock(&hugetlb_lock)
> >     spin_lock(&hugetlb_lock)
> >     clear_page_huge_active(page)
> >     enqueue_huge_page(page)
> >     // It is wrong, the page is already freed
> >     spin_unlock(&hugetlb_lock)
> >
> > The race windows is between put_page() and spin_lock() which
> > is in the __free_huge_page().
>
> The race window reall is between put_page and dissolve_free_huge_page.
> And the result is that the put_page path would clobber an unrelated page
> (either free or already reused page) which is quite serious.
> Fortunatelly pages are dissolved very rarely. I believe that user would
> require to be privileged to hit this by intention.
>
> > We should make sure that the page is already on the free list
> > when it is dissolved.
>
> Another option would be to check for PageHuge in __free_huge_page. Have
> you considered that rather than add yet another state? The scope of the
> spinlock would have to be extended. If that sounds more tricky then can
> we check the page->lru in the dissolve path? If the page is still
> PageHuge and reference count 0 then there shouldn't be many options
> where it can be queued, right?

Did you mean that we iterate over the free list to check whether
the page is on the free list? If so, I do not think it is a good solution
than introducing another state. Because if there are a lot of pages
on the free list, it may take some time to do it with holding
hugetlb_lock. Right? Actually, we have some tail page structs
to store the state. At least it's not in short supply right now.

Thanks.

>
> > Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle hugepage")
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  mm/hugetlb.c | 38 ++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 38 insertions(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 4741d60f8955..8ff138c17129 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -79,6 +79,21 @@ DEFINE_SPINLOCK(hugetlb_lock);
> >  static int num_fault_mutexes;
> >  struct mutex *hugetlb_fault_mutex_table ____cacheline_aligned_in_smp;
> >
> > +static inline bool PageHugeFreed(struct page *head)
> > +{
> > +     return (unsigned long)head[3].mapping == -1U;
> > +}
> > +
> > +static inline void SetPageHugeFreed(struct page *head)
> > +{
> > +     head[3].mapping = (void *)-1U;
> > +}
> > +
> > +static inline void ClearPageHugeFreed(struct page *head)
> > +{
> > +     head[3].mapping = NULL;
> > +}
> > +
> >  /* Forward declaration */
> >  static int hugetlb_acct_memory(struct hstate *h, long delta);
> >
> > @@ -1028,6 +1043,7 @@ static void enqueue_huge_page(struct hstate *h, struct page *page)
> >       list_move(&page->lru, &h->hugepage_freelists[nid]);
> >       h->free_huge_pages++;
> >       h->free_huge_pages_node[nid]++;
> > +     SetPageHugeFreed(page);
> >  }
> >
> >  static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid)
> > @@ -1044,6 +1060,7 @@ static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid)
> >
> >               list_move(&page->lru, &h->hugepage_activelist);
> >               set_page_refcounted(page);
> > +             ClearPageHugeFreed(page);
> >               h->free_huge_pages--;
> >               h->free_huge_pages_node[nid]--;
> >               return page;
> > @@ -1291,6 +1308,17 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> >                                               unsigned int order) { }
> >  #endif
> >
> > +/*
> > + * Because we reuse the mapping field of some tail page structs, we should
> > + * reset those mapping to initial value before @head is freed to the buddy
> > + * allocator. The invalid value will be checked in the free_tail_pages_check().
> > + */
> > +static inline void reset_tail_page_mapping(struct hstate *h, struct page *head)
> > +{
> > +     if (!hstate_is_gigantic(h))
> > +             head[3].mapping = TAIL_MAPPING;
> > +}
> > +
> >  static void update_and_free_page(struct hstate *h, struct page *page)
> >  {
> >       int i;
> > @@ -1298,6 +1326,7 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> >       if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> >               return;
> >
> > +     reset_tail_page_mapping(h, page);
> >       h->nr_huge_pages--;
> >       h->nr_huge_pages_node[page_to_nid(page)]--;
> >       for (i = 0; i < pages_per_huge_page(h); i++) {
> > @@ -1504,6 +1533,7 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> >       spin_lock(&hugetlb_lock);
> >       h->nr_huge_pages++;
> >       h->nr_huge_pages_node[nid]++;
> > +     ClearPageHugeFreed(page);
> >       spin_unlock(&hugetlb_lock);
> >  }
> >
> > @@ -1770,6 +1800,14 @@ int dissolve_free_huge_page(struct page *page)
> >               int nid = page_to_nid(head);
> >               if (h->free_huge_pages - h->resv_huge_pages == 0)
> >                       goto out;
> > +
> > +             /*
> > +              * We should make sure that the page is already on the free list
> > +              * when it is dissolved.
> > +              */
> > +             if (unlikely(!PageHugeFreed(head)))
> > +                     goto out;
> > +
> >               /*
> >                * Move PageHWPoison flag from head page to the raw error page,
> >                * which makes any subpages rather than the error page reusable.
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs