Re: [patch] NULL pointer check for vma->vm_mm

[Kenichi Okuyama]

Dear Andrew, all,

First of all, thank you for looking at the patch.

I do agree that if mm is NULL, system will call Oops anyway.
However, since it's oops, it does not stop the system, nor call kdump.

By calling BUG_ON(), it'll gives us chance of calling kdump at the first chance.

Since this is very rare to happen, I thought we should capture the incident
whenever possible. On other hand, because BUG_ON macro is very light,
I thought this will not harm any performance...

Forgive me in advance if I was wrong.
I still think checking mm with BUG_ON here is better than counting on Oops.

best regards,



2008/2/1, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> On Fri, 1 Feb 2008 16:39:07 +0900 "Kenichi Okuyama" <kenichi.okuyama@xxxxxxxxx> 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.
>
>


-- 
奥山　健一(Kenichi Okuyama) [煤背会: No. 0x00000001]
URL: http://www.dd.iij4u.or.jp/~okuyamak/
     http://developer.osdl.jp/projects/doubt/
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