[PATCH v2] mm: fix account pmd page to the process

[PATCH v2] mm: fix account pmd page to the process

[zhongjiang]

From: zhong jiang <zhongjiang@huawei.com>

huge_pmd_share accounts the number of pmds incorrectly when it races
with a parallel pud instantiation. vma_interval_tree_foreach will
increase the counter but then has to recheck the pud with the pte lock
held and the back off path should drop the increment. The previous
code would lead to an elevated pmd count which shouldn't be very
harmful (check_mm() might complain and oom_badness() might be marginally
confused) but this is worth fixing.

Suggested-off-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
---
 mm/hugetlb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 19d0d08..3072857 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4191,7 +4191,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
 				(pmd_t *)((unsigned long)spte & PAGE_MASK));
 	} else {
 		put_page(virt_to_page(spte));
-		mm_inc_nr_pmds(mm);
+		mm_dec_nr_pmds(mm);
 	}
 	spin_unlock(ptl);
 out:
-- 
1.8.3.1

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Re: [PATCH v2] mm: fix account pmd page to the process

[Michal Hocko]

On Fri 17-06-16 21:13:12, zhongjiang wrote:
> From: zhong jiang <zhongjiang@huawei.com>
> 
> huge_pmd_share accounts the number of pmds incorrectly when it races
> with a parallel pud instantiation. vma_interval_tree_foreach will
> increase the counter but then has to recheck the pud with the pte lock
> held and the back off path should drop the increment. The previous
> code would lead to an elevated pmd count which shouldn't be very
> harmful (check_mm() might complain and oom_badness() might be marginally
> confused) but this is worth fixing.

Kirill has posted a patch which is imho better [1]

[1] http://lkml.kernel.org/r/20160617122506.GC6534@node.shutemov.name
-- 
Michal Hocko
SUSE Labs

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