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

[Michal Hocko]

On Wed 06-01-21 12:58:29, Mike Kravetz wrote:
> On 1/6/21 8:56 AM, Michal Hocko 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?
> 
> The tricky part with expanding lock scope will be the potential call to
> hugepage_subpool_put_pages as it may also try to acquire the hugetlb_lock.

Can we rearrange the code and move hugepage_subpool_put_pages after all
this is done? Or is there any strong reason for the particular ordering?

> I am not sure what you mean by 'check the page->lru'?  If we knew the page
> was on the free list, then we could dissolve.  But, I do not think there
> is an easy way to determine that from page->lru.  A hugetlb page is either
> going to be on the active list or free list.

Can it be on the active list with ref count = 0?
-- 
Michal Hocko
SUSE Labs