linux-kernel - Re: [RFC][PATCH v2] slub: Keep page and object in sync in slab_alloc

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Date:	Fri, 18 Jan 2013 14:44:55 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	RT <linux-rt-users@...r.kernel.org>,
	Clark Williams <clark@...hat.com>,
	John Kacur <jkacur@...il.com>,
	"Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [RFC][PATCH v2] slub: Keep page and object in sync in
 slab_alloc_node()

On Thu, 17 Jan 2013, Steven Rostedt wrote:

> In slab_alloc_node(), after the cpu_slab is assigned, if the task is
> preempted and moves to another CPU, there's nothing keeping the page and
> object in sync. The -rt kernel crashed because page was NULL and object
> was not, and the node_match() dereferences page. Even though the crash
> happened on -rt, there's nothing that's keeping this from happening on
> mainline.
>
> The easiest fix is to disable interrupts for the entire time from
> acquiring the current CPU cpu_slab and assigning the object and page.
> After that, it's fine to allow preemption.

Its easiest to just check for the NULL pointer as initally done. The call
to __slab_alloc can do what the fastpath does.

And the fastpath will verify that the c->page pointer was not changed.
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