From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx125.postini.com [74.125.245.125]) by kanga.kvack.org (Postfix) with SMTP id 965466B002C for ; Thu, 1 Mar 2012 02:30:39 -0500 (EST) Received: by dakp5 with SMTP id p5so503320dak.8 for ; Wed, 29 Feb 2012 23:30:38 -0800 (PST) Subject: Re: [PATCH -next] slub: set PG_slab on all of slab pages From: Namhyung Kim In-Reply-To: References: <1330505674-31610-1-git-send-email-namhyung.kim@lge.com> Content-Type: text/plain; charset="UTF-8" Date: Thu, 01 Mar 2012 16:30:31 +0900 Message-ID: <1330587031.1762.46.camel@leonhard> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: Namhyung Kim , Pekka Enberg , Matt Mackall , linux-mm@kvack.org, linux-kernel@vger.kernel.org Hi, 2012-02-29, 09:24 -0600, Christoph Lameter wrote: > On Wed, 29 Feb 2012, Namhyung Kim wrote: > > > Unlike SLAB, SLUB doesn't set PG_slab on tail pages, so if a user would > > call free_pages() incorrectly on a object in a tail page, she will get > > confused with the undefined result. Setting the flag would help her by > > emitting a warning on bad_page() in such a case. > > NAK > > You cannot free a tail page of a compound higher order page independently. > You must free the whole compound. > I meant freeing a *slab object* resides in a compound page using buddy system API (e.g. free_pages). I know it's definitely a programming error. However there's no safety net to protect and/or warn such a misbehavior AFAICS - except for head page which has PG_slab set - when it happened by any chance. Without it, it might be possible to free part of tail pages silently, and cause unexpected not-so-funny results some time later. It should be hard to find out. When I ran such a bad code using SLAB, I was able to be notified immediately. That's why I'd like to add this patch to SLUB too. In addition, it will give more correct value for slab pages when using /proc/kpageflags IMHO. -- Regards, Namhyung Kim -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org