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

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Message-ID: <20120306011624.GA14274@barrios>
Date:	Tue, 6 Mar 2012 10:16:24 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Namhyung Kim <namhyung@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] slub: set PG_slab on all of slab pages

Hi Christoph,

On Mon, Mar 05, 2012 at 08:48:33AM -0600, Christoph Lameter wrote:
> On Sun, 4 Mar 2012, Minchan Kim wrote:
> 
> > I read this thread and I feel the we don't reach right point.
> > I think it's not a compound page problem.
> > We can face above problem where we allocates big order page without __GFP_COMP
> > and free middle page of it.
> 
> Yes we can do that and doing such a thing seems to be more legitimate
> since one could argue that the user did not request an atomic allocation
> unit from the page allocator and therefore the freeing of individual
> pages in that group is permissible. If memory serves me right we do that
> sometimes.

To be leitimate, user have to handle subpages's ref counter well.
But I think it's not desirable. If user want it, he should use
split_page instead of modifying ref counter directly.

> 
> However if compound pages are requested then such an atomic allocation
> unit *was* requested and the page allocator should not allow to free
> individual pages.

Yes. In fact, I am not sure this problem is related to compound page.
If it is compound page, tail page's ref count should be zero.
When user calls __free_pages in tail page by mistake, it should not pass
into __free_pages_ok but reference count would be underflow.
Later, when head page is freed, we could catch it in free_pages_check.

So I had a question to Namhyung that he can see bad page message by PG_slab when he uses SLUB
with his patch. If the problem still happens, something seems to modify tail page's ref count
directly without get_page. It's apparently BUG.

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