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
