linux-kernel - Re: [patch] NULL pointer check for vma->vm

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Message-Id: <20080131235544.346b938a.akpm@linux-foundation.org>
Date:	Thu, 31 Jan 2008 23:55:44 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kenichi Okuyama" <kenichi.okuyama@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] NULL pointer check for vma->vm_mm

On Fri, 1 Feb 2008 16:39:07 +0900 "Kenichi Okuyama" <kenichi.okuyama@...il.com> wrote:

> Dear all,
> 
> I was looking at the ./mm/rmap.c .. I found that, in function
> "page_referenced_one()",
>    struct mm_struct *mm = vma->vm_mm;
> was being refererred without NULL check.
> 
> Though I do agree that this works for most of the cases, I thought it
> is better to add
> BUG_ON() for case of mm being NULL.
> 
> attached is the patch for this

If we dereference NULL then the kernel will display basically the same
information as would a BUG, and it takes the same action.  So adding a
BUG_ON here really doesn't gain us anything.

Also, I think vma->vm_mm == 0 is not a valid state, so this just shouldn't
happen - the code is OK to assume that a particular invariant is being
honoured.

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