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

[Muchun Song]

On Thu, Jan 7, 2021 at 8:38 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 07-01-21 19:38:00, Muchun Song wrote:
> > On Thu, Jan 7, 2021 at 7:18 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Thu 07-01-21 16:53:13, Muchun Song wrote:
> > > > On Thu, Jan 7, 2021 at 4:41 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > >
> > > > > 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.
> > > >
> > > > The page can be on the free list or active list or empty when it
> > > > is freed to the pool. How to check whether it is on the free list?
> > >
> > > As I've said, I might be missing something here. But if the page is
> > > freed why does it matter whether it is on a active list or free list
> > > from the dissolve operation POV?
> >
> > As you said "check the page->lru". I have a question.
> > How to check the page->lru in the dissolve path?
>
> list_empty?

No.

>
> > BTW, dissolve_free_huge_page aims to free the page
> > to buddy allocator. put_page (for HugeTLB page) aims
> > to free the page to the hugepage pool.
>
> Right. Can we simply back off in the dissolving path when ref count is
> 0 && PageHuge() if list_empty(page->lru)? Is there any other scenario
> when the all above is true and the page is not being freed?

The list_empty(&page->lru) may always return false.
The page before freeing is on the active list
(hstate->hugepage_activelist).Then it is on the free list
after freeing. So list_empty(&page->lru) is always false.

> --
> Michal Hocko
> SUSE Labs