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

[Muchun Song]

On Wed, Jan 6, 2021 at 7:22 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 1/4/21 6:55 PM, Muchun Song wrote:
> > On Tue, Jan 5, 2021 at 8:02 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >>> ---
> >>>  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.
>
> Just FYI,
> I went back and explored the option of doing set_page_huge_active
> when a page was put on the active list and clear_page_huge_active
> when put on the free list.  This would be much like what you are
> doing with PageHugeFreed.  Commit bcc54222309c which added page_huge_active
> implied that this was possible.  Then I remembered a race fixed in
> cb6acd01e2e4 that required delaying the call to set_page_huge_active
> in hugetlb_no_page.  So, such a scheme would not work.

Sounds like a tortuous story. :)

>
> Also,
> It seems we could use head[3].mapping for PageHugeFreed ?  Not much
> of an advantage.  It does not add another tail page needed to store
> page metadata.  And, this fits within the already defined
> HUGETLB_CGROUP_MIN_ORDER.

It is fine to me. Will do. Thanks.

> --
> Mike Kravetz