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
