Re: [PATCH] mm: hugetlb: fix a race between memory-failure/soft_offline and gather_surplus_pages

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Wed, Apr 21, 2021 at 11:03:24AM -0700, Mike Kravetz wrote:
> On 4/21/21 1:33 AM, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Wed, Apr 21, 2021 at 10:03:34AM +0200, Michal Hocko wrote:
> >> [Cc Naoya]
> >>
> >> On Wed 21-04-21 14:02:59, Muchun Song wrote:
> >>> The possible bad scenario:
> >>>
> >>> CPU0:                           CPU1:
> >>>
> >>>                                 gather_surplus_pages()
> >>>                                   page = alloc_surplus_huge_page()
> >>> memory_failure_hugetlb()
> >>>   get_hwpoison_page(page)
> >>>     __get_hwpoison_page(page)
> >>>       get_page_unless_zero(page)
> >>>                                   zero = put_page_testzero(page)
> >>>                                   VM_BUG_ON_PAGE(!zero, page)
> >>>                                   enqueue_huge_page(h, page)
> >>>   put_page(page)
> >>>
> >>> The refcount can possibly be increased by memory-failure or soft_offline
> >>> handlers, we can trigger VM_BUG_ON_PAGE and wrongly add the page to the
> >>> hugetlb pool list.
> >>
> >> The hwpoison side of this looks really suspicious to me. It shouldn't
> >> really touch the reference count of hugetlb pages without being very
> >> careful (and having hugetlb_lock held).
> > 
> > I have the same feeling, there is a window where a hugepage is refcounted
> > during converting from buddy free pages into free hugepage, so refcount
> > alone is not enough to prevent the race.  hugetlb_lock is retaken after
> > alloc_surplus_huge_page returns, so simply holding hugetlb_lock in
> > get_hwpoison_page() seems not work.  Is there any status bit to show that a
> > hugepage is just being initialized (not in free hugepage pool or in use)?
> > 
> 
> It seems we can also race with the code that makes a compound page a
> hugetlb page.  The memory failure code could be called after allocating
> pages from buddy and before setting compound page DTOR.  So, the memory
> handling code will process it as a compound page.

Yes, so get_hwpoison_page() has to call get_page_unless_zero()
only when memory_failure() can surely handle the error.

> 
> Just thinking that this may not be limited to the hugetlb specific memory
> failure handling?

Currently hugetlb page is the only type of compound page supported by memory
failure.  But I agree with you that other types of compound pages have the
same race window, and judging only with get_page_unless_zero() is dangerous.
So I think that __get_hwpoison_page() should have the following structure:

  if (PageCompound) {
      if (PageHuge) {
          if (PageHugeFreed || PageHugeActive) {
              if (get_page_unless_zero)
                  return 0;   // path for in-use hugetlb page
              else
                  return 1;   // path for free hugetlb page
          } else {
              return -EBUSY;  // any transient hugetlb page
          }
      } else {
          ... // any other compound page (like thp, slab, ...)
      }
  } else {
      ...   // any non-compound page
  }

Thanks,
Naoya Horiguchi