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

[Michal Hocko]

On Thu 07-01-21 13:39:38, Muchun Song wrote:
> On Thu, Jan 7, 2021 at 12:56 AM Michal Hocko <mhocko@xxxxxxxx> 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?

No I meant to check that the page is enqueued which along with ref count
= 0 should mean it has been released to the pool unless I am missing
something.
-- 
Michal Hocko
SUSE Labs