linux-kernel - Re: [PATCH] fix page_reset_bad

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Message-Id: <20080418123653.0234dc98.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 18 Apr 2008 12:36:53 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, shiwh@...fujitsu.com,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"xemul@...nvz.org" <xemul@...nvz.org>
Subject: Re: [PATCH] fix page_reset_bad_cgroup

On Thu, 17 Apr 2008 20:22:21 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Fri, 18 Apr 2008 12:10:46 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > page->page_cgroup is unsigned long. Should be cleared by 0UL.
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > 
> > 
> > Index: linux-2.6.25/include/linux/memcontrol.h
> > ===================================================================
> > --- linux-2.6.25.orig/include/linux/memcontrol.h
> > +++ linux-2.6.25/include/linux/memcontrol.h
> > @@ -30,7 +30,7 @@ struct mm_struct;
> >  extern void mm_init_cgroup(struct mm_struct *mm, struct task_struct *p);
> >  extern void mm_free_cgroup(struct mm_struct *mm);
> >  
> > -#define page_reset_bad_cgroup(page)	((page)->page_cgroup = 0)
> > +#define page_reset_bad_cgroup(page)	((page)->page_cgroup = 0UL)
> 
> That's pretty cosmetic.
> 
> In fact it could be argued that we shouldn't do this.  "0" is zero
> regardless of the type of the LHS.  So if we later change page_cgroup to
> u32 or u16 or `struct superblock *', the old page_reset_bad_cgroup() will
> happily continue to work.  There's no need to make this code aware of
> page_cgroup's type, to this extent?
>
Ah, sorry. I was confused...please ignore.
(I debugged an user program which clear a pointer with 0, yesterday..)

-Kame



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