From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by rv-out-0910.google.com with SMTP id f1so670812rvb.26 for ; Fri, 01 Feb 2008 00:24:17 -0800 (PST) Message-ID: <3fd7d7a70802010024q22b4d179mf56e6d4b60e4f574@mail.gmail.com> Date: Fri, 1 Feb 2008 17:24:17 +0900 From: "Kenichi Okuyama" Subject: Re: [patch] NULL pointer check for vma->vm_mm In-Reply-To: <20080131235544.346b938a.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <3fd7d7a70801312339p2a142096p83ed286c81379728@mail.gmail.com> <20080131235544.346b938a.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: 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 : > On Fri, 1 Feb 2008 16:39:07 +0900 "Kenichi Okuyama" 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/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org