On Thu 07-01-21 16:52:19, Mike Kravetz wrote: > On 1/7/21 12:40 AM, Michal Hocko wrote: > > 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? > > The reservation code is so fragile, I always get nervous when making > any changes. However, the straight forward patch below passes some > simple testing. The only difference I can see is that global counts > are adjusted before sub-pool counts. This should not be an issue as > global and sub-pool counts are adjusted independently (not under the > same lock). Allocation code checks sub-pool counts before global > counts. So, there is a SMALL potential that a racing allocation which > previously succeeded would now fail. I do not think this is an issue > in practice. > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 3b38ea958e95..658593840212 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -1395,6 +1395,11 @@ static void __free_huge_page(struct page *page) > (struct hugepage_subpool *)page_private(page); > bool restore_reserve; > > + spin_lock(&hugetlb_lock); > + /* check for race with dissolve_free_huge_page/update_and_free_page */ > + if (!PageHuge(page)) > + return; > + This really wants to unlock the lock, right? But this is indeed what I've had in mind. -- Michal Hocko SUSE Labs
