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

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Message-ID: <0000013c4a69a2cf-1a19a6f6-e6a3-4f06-99a4-10fdd4b9aca2-000000@email.amazonses.com>
Date:	Thu, 17 Jan 2013 21:28:22 +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] slub: Check for page NULL before doing the node_match
 check

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;
 	}


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