linux-kernel - Re: [RFC][PATCH] slub: Check for page NULL before doing the node

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Date:	Thu, 17 Jan 2013 18:10:45 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Christoph Lameter <cl@...ux.com>
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] slub: Check for page NULL before doing the
 node_match check

On Thu, 2013-01-17 at 17:46 -0500, Steven Rostedt wrote:
> On Thu, 2013-01-17 at 21:51 +0000, Christoph Lameter wrote:
> 
> > This is dealing with the same cpu being interrupted. Some of these
> > segments are in interrupt disable sections so they are not affected.
> 
> Except that we are not always on the same CPU. Now I'm looking at
> mainline (non modified by -rt):

Because there's also nothing to keep page related to object either, we
may just need to do:


> 
> From slab_alloc_node():
> 
> 	/*
> 	 * Must read kmem_cache cpu data via this cpu ptr. Preemption is
> 	 * enabled. We may switch back and forth between cpus while
> 	 * reading from one cpu area. That does not matter as long
> 	 * as we end up on the original cpu again when doing the cmpxchg.
> 	 */

	local_irq_save(flags);

> 	c = __this_cpu_ptr(s->cpu_slab);
> 
> 	/*
> 	 * The transaction ids are globally unique per cpu and per operation on
> 	 * a per cpu queue. Thus they can be guarantee that the cmpxchg_double
> 	 * occurs on the right processor and that there was no operation on the
> 	 * linked list in between.
> 	 */
> 	tid = c->tid;
> 	barrier();
> 
> 	object = c->freelist;
> 	page = c->page;

	r = !object || !node_match(page, node);
	local_irq_restore(flags);

	if (unlikely(r)) {

> 		object = __slab_alloc(s, gfpflags, node, addr, c);
> 

I was thinking at first we could use preempt_disable(), but if an
interrupt comes in after we set object = c->freelist, and changes
c->page, then we disassociate the freelist and page again.

-- Steve


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