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Date: Thu, 1 Mar 2012 09:03:16 -0600 (CST)
From: Christoph Lameter <cl@gentwo.org>
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To: Namhyung Kim <namhyung@gmail.com>
cc: Namhyung Kim <namhyung.kim@lge.com>, Pekka Enberg <penberg@kernel.org>,
        Matt Mackall <mpm@selenic.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] slub: set PG_slab on all of slab pages
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On Thu, 1 Mar 2012, Namhyung Kim wrote:

> > 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.

?? One generally passed a struct page pointer to the page allocator. Slab
allocator takes pointers to object. The calls that take a pointer to an
object must have a page aligned value.

> 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.

Ok then fix the page allocator to BUG() on tail pages. That is an issue
with the page allocator not the slab allocator.

Adding PG_tail to the flags checked on free should do the trick (at least
for 64 bit).