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Subject: Re: [RFC][PATCH] slub: Check for page NULL before doing the
 node_match check
From: Eric Dumazet <eric.dumazet@gmail.com>
In-Reply-To: <0000013c4a69a2cf-1a19a6f6-e6a3-4f06-99a4-10fdd4b9aca2-000000@email.amazonses.com>
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Date: Thu, 17 Jan 2013 13:36:23 -0800
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To: Christoph Lameter <cl@linux.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>, Pekka Enberg <penberg@kernel.org>, Matt Mackall <mpm@selenic.com>, Thomas Gleixner <tglx@linutronix.de>, RT <linux-rt-users@vger.kernel.org>, Clark Williams <clark@redhat.com>, John Kacur <jkacur@gmail.com>, "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>

On Thu, 2013-01-17 at 21:28 +0000, Christoph Lameter wrote:
> On Thu, 17 Jan 2013, Steven Rostedt wrote:
> 
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -2399,7 +2399,7 @@ redo:
> > >
> > >  	object = c->freelist;
> > >  	page = c->page;
> > > -	if (unlikely(!object || !node_match(page, node)))
> > > +	if (unlikely(!object || !page || !node_match(page, node)))
> >
> > I'm still trying to see if c->freelist != NULL and c->page == NULL isn't
> > a bug. The cmpxchg_doubles are a little confusing. If it's not expected
> > that page is NULL but freelist isn't than we need to figure out why it
> > happened.
> 
> hmmm.. We may want to change the sequence of updates to c->page and
> c->freelist. Update c->freelist to be NULL first so that we always enter
> the slow path for these cases where we can do more expensive
> synchronization.
> 
> Index: linux/mm/slub.c
> ===================================================================
> --- linux.orig/mm/slub.c	2013-01-15 10:42:08.490183607 -0600
> +++ linux/mm/slub.c	2013-01-17 15:27:48.973051155 -0600
> @@ -1993,8 +1993,8 @@ static inline void flush_slab(struct kme
>  	deactivate_slab(s, c->page, c->freelist);
> 
>  	c->tid = next_tid(c->tid);
> -	c->page = NULL;
>  	c->freelist = NULL;
> +	c->page = NULL;
>  }
> 
>  /*
> @@ -2227,8 +2227,8 @@ redo:
>  	if (unlikely(!node_match(page, node))) {
>  		stat(s, ALLOC_NODE_MISMATCH);
>  		deactivate_slab(s, page, c->freelist);
> -		c->page = NULL;
>  		c->freelist = NULL;
> +		c->page = NULL;
>  		goto new_slab;
>  	}
> 
> @@ -2239,8 +2239,8 @@ redo:
>  	 */
>  	if (unlikely(!pfmemalloc_match(page, gfpflags))) {
>  		deactivate_slab(s, page, c->freelist);
> -		c->page = NULL;
>  		c->freelist = NULL;
> +		c->page = NULL;
>  		goto new_slab;
>  	}
> 

Without appropriate barriers, this change is a shoot in the dark.





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