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

[Mike Kravetz]

On 06/17/2016 05:25 AM, Kirill A. Shutemov wrote:
> 
> From fd22922e7b4664e83653a84331f0a95b985bff0c Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Date: Fri, 17 Jun 2016 15:07:03 +0300
> Subject: [PATCH] hugetlb: fix nr_pmds accounting with shared page tables
> 
> We account HugeTLB's shared page table to all processes who share it.
> The accounting happens during huge_pmd_share().
> 
> If somebody populates pud entry under us, we should decrease pagetable's
> refcount and decrease nr_pmds of the process.
> 
> By mistake, I increase nr_pmds again in this case. :-/
> It will lead to "BUG: non-zero nr_pmds on freeing mm: 2" on process'
> exit.
> 
> Let's fix this by increasing nr_pmds only when we're sure that the page
> table will be used.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Nice,
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

I agree that we do not necessarily need a back port.  I have not seen
reports of people experiencing this race and seeing the BUG (on mm
tear-down).

zhongjiang, did someone actually hit the BUG?  Or, did you find it by
code examination?

-- 
Mike Kravetz

> Reported-by: zhongjiang <zhongjiang@xxxxxxxxxx>
> Fixes: dc6c9a35b66b ("mm: account pmd page tables to the process")
> Cc: <stable@xxxxxxxxxxxxxxx>        [4.0+]
> ---
>  mm/hugetlb.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e197cd7080e6..ed6a537f0878 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4216,7 +4216,6 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
>  		if (saddr) {
>  			spte = huge_pte_offset(svma->vm_mm, saddr);
>  			if (spte) {
> -				mm_inc_nr_pmds(mm);
>  				get_page(virt_to_page(spte));
>  				break;
>  			}
> @@ -4231,9 +4230,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
>  	if (pud_none(*pud)) {
>  		pud_populate(mm, pud,
>  				(pmd_t *)((unsigned long)spte & PAGE_MASK));
> +		mm_inc_nr_pmds(mm);
>  	} else {
>  		put_page(virt_to_page(spte));
> -		mm_inc_nr_pmds(mm);
>  	}
>  	spin_unlock(ptl);
>  out:
>