Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardownof hugetlbfs shared page tables V2 (resend)

[Larry Woodman]

On 07/26/2012 11:48 PM, Larry Woodman wrote:


Mel, did you see this???

Larry

> > This patch looks good to me.
> >
> > Larry, does Hugh's patch survive your testing?
>
> Like I said earlier, no.  However, I finally set up a reproducer that 
> only takes a few seconds
> on a large system and this totally fixes the problem:
>
> ------------------------------------------------------------------------------------------------------------------------- 
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index c36febb..cc023b8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2151,7 +2151,7 @@ int copy_hugetlb_page_range(struct mm_struct 
> *dst, struct mm_struct *src,
>                         goto nomem;
>
>                 /* If the pagetables are shared don't copy or take 
> references */
> -               if (dst_pte == src_pte)
> +               if (*(unsigned long *)dst_pte == *(unsigned long 
> *)src_pte)
>                         continue;
>
>                 spin_lock(&dst->page_table_lock);
> --------------------------------------------------------------------------------------------------------------------------- 
>
>
> When we compare what the src_pte & dst_pte point to instead of their 
> addresses everything works,
> I suspect there is a missing memory barrier somewhere ???
>
> Larry

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