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Wed, 06 Jan 2021 21:00:36 +0000 Received: from pps.filterd (userp3020.oracle.com [127.0.0.1]) by userp3020.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 106KuQdq079426; Wed, 6 Jan 2021 20:58:35 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by userp3020.oracle.com with ESMTP id 35w3qsje2h-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 06 Jan 2021 20:58:35 +0000 Received: from abhmp0008.oracle.com (abhmp0008.oracle.com [141.146.116.14]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 106KwU25029412; Wed, 6 Jan 2021 20:58:31 GMT Received: from [192.168.2.112] (/50.38.35.18) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 06 Jan 2021 12:58:30 -0800 Subject: Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page To: Michal Hocko , Muchun Song Cc: akpm@linux-foundation.org, n-horiguchi@ah.jp.nec.com, ak@linux.intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20210106084739.63318-1-songmuchun@bytedance.com> <20210106084739.63318-4-songmuchun@bytedance.com> <20210106165632.GT13207@dhcp22.suse.cz> From: Mike Kravetz Message-ID: Date: Wed, 6 Jan 2021 12:58:29 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <20210106165632.GT13207@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9856 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 malwarescore=0 mlxscore=0 spamscore=0 mlxlogscore=999 phishscore=0 bulkscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101060120 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9856 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 bulkscore=0 spamscore=0 impostorscore=0 phishscore=0 lowpriorityscore=0 suspectscore=0 priorityscore=1501 mlxscore=0 malwarescore=0 clxscore=1015 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101060120 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. 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. > >> Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle hugepage") >> Signed-off-by: Muchun Song >> --- >> mm/hugetlb.c | 38 ++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 38 insertions(+) >> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index 4741d60f8955..8ff138c17129 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -79,6 +79,21 @@ DEFINE_SPINLOCK(hugetlb_lock); >> static int num_fault_mutexes; >> struct mutex *hugetlb_fault_mutex_table ____cacheline_aligned_in_smp; >> >> +static inline bool PageHugeFreed(struct page *head) >> +{ >> + return (unsigned long)head[3].mapping == -1U; >> +} >> + >> +static inline void SetPageHugeFreed(struct page *head) >> +{ >> + head[3].mapping = (void *)-1U; >> +} >> + >> +static inline void ClearPageHugeFreed(struct page *head) >> +{ >> + head[3].mapping = NULL; >> +} >> + >> /* Forward declaration */ >> static int hugetlb_acct_memory(struct hstate *h, long delta); >> >> @@ -1028,6 +1043,7 @@ static void enqueue_huge_page(struct hstate *h, struct page *page) >> list_move(&page->lru, &h->hugepage_freelists[nid]); >> h->free_huge_pages++; >> h->free_huge_pages_node[nid]++; >> + SetPageHugeFreed(page); >> } >> >> static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid) >> @@ -1044,6 +1060,7 @@ static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid) >> >> list_move(&page->lru, &h->hugepage_activelist); >> set_page_refcounted(page); >> + ClearPageHugeFreed(page); >> h->free_huge_pages--; >> h->free_huge_pages_node[nid]--; >> return page; >> @@ -1291,6 +1308,17 @@ static inline void destroy_compound_gigantic_page(struct page *page, >> unsigned int order) { } >> #endif >> >> +/* >> + * Because we reuse the mapping field of some tail page structs, we should >> + * reset those mapping to initial value before @head is freed to the buddy >> + * allocator. The invalid value will be checked in the free_tail_pages_check(). >> + */ When I suggested using head[3].mapping for this state, I was not aware of this requirement. My suggestion was only following the convention used in PageHugeTemporary. I would not have made the suggestion if I had realized this was required. Sorry. -- Mike Kravetz >> +static inline void reset_tail_page_mapping(struct hstate *h, struct page *head) >> +{ >> + if (!hstate_is_gigantic(h)) >> + head[3].mapping = TAIL_MAPPING; >> +} >> + >> static void update_and_free_page(struct hstate *h, struct page *page) >> { >> int i; >> @@ -1298,6 +1326,7 @@ static void update_and_free_page(struct hstate *h, struct page *page) >> if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported()) >> return; >> >> + reset_tail_page_mapping(h, page); >> h->nr_huge_pages--; >> h->nr_huge_pages_node[page_to_nid(page)]--; >> for (i = 0; i < pages_per_huge_page(h); i++) { >> @@ -1504,6 +1533,7 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid) >> spin_lock(&hugetlb_lock); >> h->nr_huge_pages++; >> h->nr_huge_pages_node[nid]++; >> + ClearPageHugeFreed(page); >> spin_unlock(&hugetlb_lock); >> } >> >> @@ -1770,6 +1800,14 @@ int dissolve_free_huge_page(struct page *page) >> int nid = page_to_nid(head); >> if (h->free_huge_pages - h->resv_huge_pages == 0) >> goto out; >> + >> + /* >> + * We should make sure that the page is already on the free list >> + * when it is dissolved. >> + */ >> + if (unlikely(!PageHugeFreed(head))) >> + goto out; >> + >> /* >> * Move PageHWPoison flag from head page to the raw error page, >> * which makes any subpages rather than the error page reusable. >> -- >> 2.11.0 >