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Message-ID: <509ef752-cc2e-3edf-5871-87f971a7bc0f@oracle.com>
Date: Thu, 7 Jan 2021 16:52:19 -0800
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Muchun Song <songmuchun@...edance.com>, 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 1/7/21 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;
+
VM_BUG_ON_PAGE(page_count(page), page);
VM_BUG_ON_PAGE(page_mapcount(page), page);
@@ -1403,26 +1408,6 @@ static void __free_huge_page(struct page *page)
restore_reserve = PagePrivate(page);
ClearPagePrivate(page);
- /*
- * If PagePrivate() was set on page, page allocation consumed a
- * reservation. If the page was associated with a subpool, there
- * would have been a page reserved in the subpool before allocation
- * via hugepage_subpool_get_pages(). Since we are 'restoring' the
- * reservtion, do not call hugepage_subpool_put_pages() as this will
- * remove the reserved page from the subpool.
- */
- if (!restore_reserve) {
- /*
- * A return code of zero implies that the subpool will be
- * under its minimum size if the reservation is not restored
- * after page is free. Therefore, force restore_reserve
- * operation.
- */
- if (hugepage_subpool_put_pages(spool, 1) == 0)
- restore_reserve = true;
- }
-
- spin_lock(&hugetlb_lock);
clear_page_huge_active(page);
hugetlb_cgroup_uncharge_page(hstate_index(h),
pages_per_huge_page(h), page);
@@ -1446,6 +1431,28 @@ static void __free_huge_page(struct page *page)
enqueue_huge_page(h, page);
}
spin_unlock(&hugetlb_lock);
+
+ /*
+ * If PagePrivate() was set on page, page allocation consumed a
+ * reservation. If the page was associated with a subpool, there
+ * would have been a page reserved in the subpool before allocation
+ * via hugepage_subpool_get_pages(). Since we are 'restoring' the
+ * reservtion, do not call hugepage_subpool_put_pages() as this will
+ * remove the reserved page from the subpool.
+ */
+ if (!restore_reserve) {
+ /*
+ * A return code of zero implies that the subpool will be
+ * under its minimum size if the reservation is not restored
+ * after page is free. Therefore, we need to add 1 to the
+ * global reserve count.
+ */
+ if (hugepage_subpool_put_pages(spool, 1) == 0) {
+ spin_lock(&hugetlb_lock);
+ h->resv_huge_pages++;
+ spin_unlock(&hugetlb_lock);
+ }
+ }
}
/*
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