linux-kernel - Re: [PATCH -next] slub: set PG

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Date:	Thu, 01 Mar 2012 16:30:31 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	Namhyung Kim <namhyung.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] slub: set PG_slab on all of slab pages

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

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