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

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  [PATCH] mm: fix account pmd page to the process zhongjiang <zhongjiang@huawei.com> - 2016-06-17 14:00 +0200
    Re: [PATCH] mm: fix account pmd page to the process Michal Hocko <mhocko@kernel.org> - 2016-06-17 14:30 +0200
      Re: [PATCH] mm: fix account pmd page to the process zhong jiang <zhongjiang@huawei.com> - 2016-06-17 15:10 +0200

#1424976 — [PATCH] mm: fix account pmd page to the process

From: zhong jiang <zhongjiang@huawei.com>

hen a process acquire a pmd table shared by other process, we
increase the account to current process. otherwise, a race result
in other tasks have set the pud entry. so it no need to increase it.

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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#1425016

On Fri 17-06-16 19:56:15, zhongjiang wrote:
> From: zhong jiang <zhongjiang@huawei.com>
> 
> hen a process acquire a pmd table shared by other process, we
> increase the account to current process. otherwise, a race result
> in other tasks have set the pud entry. so it no need to increase it.

I have really hard time to understand (well even to parse) the
changelog. What do you think about the following?
"
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.
"

But please note that I am still not 100% sure the race is real.

> 
> 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

-- 
Michal Hocko
SUSE Labs

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#1425051

On 2016/6/17 20:21, Michal Hocko wrote:
> On Fri 17-06-16 19:56:15, zhongjiang wrote:
>> From: zhong jiang <zhongjiang@huawei.com>
>>
>> hen a process acquire a pmd table shared by other process, we
>> increase the account to current process. otherwise, a race result
>> in other tasks have set the pud entry. so it no need to increase it.
> I have really hard time to understand (well even to parse) the
> changelog. What do you think about the following?
> "
> 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.
>
> "
  Yes, it is better , thanks.
> But please note that I am still not 100% sure the race is real.
   we can not completely rule out the possibility of  race,  such implementation is common
    in the kernel.   The stability of the kernel will be guaranteed.
  
   Thanks
    zhongjiang
 
>> 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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