linux-kernel - Re: [PATCH 5/5] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages

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Message-ID: <000001394596bd69-2c16d7fb-71b5-4009-95cc-7068103b2bfd-000000@email.amazonses.com>
Date:	Mon, 20 Aug 2012 19:51:10 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Mel Gorman <mgorman@...e.de>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Jones <davej@...hat.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Andi Kleen <ak@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 5/5] mempolicy: fix a memory corruption by refcount
 imbalance in alloc_pages_vma()

On Mon, 20 Aug 2012, Mel Gorman wrote:

> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 45f9825..82e872f 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1545,15 +1545,28 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
>  		struct vm_area_struct *vma, unsigned long addr)
>  {
>  	struct mempolicy *pol = task->mempolicy;
> +	int got_ref;

New variable. Need to set it to zero?

>
>  	if (vma) {
>  		if (vma->vm_ops && vma->vm_ops->get_policy) {
>  			struct mempolicy *vpol = vma->vm_ops->get_policy(vma,
>  									addr);
> -			if (vpol)
> +			if (vpol) {
>  				pol = vpol;
> -		} else if (vma->vm_policy)
> +				got_ref = 1;

Set the new variable. But it was not initialzed before. So now its 1 or
undefined?

> +			}
> +		} else if (vma->vm_policy) {
>  			pol = vma->vm_policy;
> +
> +			/*
> +			 * shmem_alloc_page() passes MPOL_F_SHARED policy with
> +			 * a pseudo vma whose vma->vm_ops=NULL. Take a reference
> +			 * count on these policies which will be dropped by
> +			 * mpol_cond_put() later
> +			 */
> +			if (mpol_needs_cond_ref(pol))
> +				mpol_get(pol);
> +		}
>  	}
>  	if (!pol)
>  		pol = &default_policy;
>

I do not see any use of got_ref. Can we get rid of the variable?


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