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

Muchun Song
2021-01-05T02:56:30Z

On Tue, Jan 5, 2021 at 8:02 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 1/3/21 10:58 PM, 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().
> >
> > We should make sure that the page is already on the free list
> > when it is dissolved.
> >
> > Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle hugepage")
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> > mm/hugetlb.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 48 insertions(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 1f3bf1710b66..72608008f8b4 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 page_private(head) == -1UL;
>
> return page_private(head + 4) == -1UL;
>
> > +}
> > +
> > +static inline void SetPageHugeFreed(struct page *head)
> > +{
> > + set_page_private(head + 4, -1UL);
> > +}
> > +
> > +static inline void ClearPageHugeFreed(struct page *head)
> > +{
> > + set_page_private(head + 4, 0);
> > +}
>
> It is unfortunate that we can not use some existing value like
> page_huge_active() to determine if dissolve_free_huge_page() should
> proceed with freeing the page to buddy. If the existing check,
>
> if (!page_count(page)) {
>
> was changed to
>
> if (!page_count(page) && !page_huge_active(page)) {
>
> the race window would be shrunk. However, the most straight forward
> way to fully close the window is with the approach taken here.

I also thought about this fix. But this is not enough. Because
we just call put_page to free the HugeTLB page without
setting activeness in some place (e.g. error handling
routines).

If we use page_huge_active, we should set activeness
before put_page. But we cannot guarantee this.

Thanks.

>
> > +
> > /* 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;
> > @@ -1504,6 +1521,7 @@ 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 +1788,36 @@ 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;
> > +
> > + /*
> > + * 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)
> > + * enqueue_huge_page(page)
> > + * // It is wrong, the page is already freed
> > + * spin_unlock(&hugetlb_lock)
> > + *
> > + * The race window is between put_page() and spin_lock() which
> > + * is in the __free_huge_page().
>
> IMO, the description of the race condition in the commit message is
> sufficient. It does not need to be here in the code. The below comment
> should be sufficient.

Thanks. Will do.

>
> --
> Mike Kravetz
>
> > + *
> > + * 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.
> >