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Date: Wed, 6 Jan 2021 12:58:29 -0800
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Michal Hocko <mhocko@...e.com>,
Muchun Song <songmuchun@...edance.com>
Cc: akpm@...ux-foundation.org, n-horiguchi@...jp.nec.com,
ak@...ux.intel.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and
dissolving the page
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 <songmuchun@...edance.com>
>> ---
>> 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
>
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