netdev - Re: [PATCH 30/31] Fix use of uninitialized variable in cache

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Message-ID: <alpine.DEB.1.00.0910020258190.25369@chino.kir.corp.google.com>
Date:	Fri, 2 Oct 2009 03:05:07 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Neil Brown <neilb@...e.de>
cc:	Suresh Jayaraman <sjayaraman@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, Miklos Szeredi <mszeredi@...e.cz>,
	Wouter Verhelst <w@...r.be>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	trond.myklebust@....uio.no
Subject: Re: [PATCH 30/31] Fix use of uninitialized variable in
 cache_grow()

On Fri, 2 Oct 2009, Neil Brown wrote:

> > > Index: mmotm/mm/slab.c
> > > ===================================================================
> > > --- mmotm.orig/mm/slab.c
> > > +++ mmotm/mm/slab.c
> > > @@ -2760,7 +2760,7 @@ static int cache_grow(struct kmem_cache
> > >  	size_t offset;
> > >  	gfp_t local_flags;
> > >  	struct kmem_list3 *l3;
> > > -	int reserve;
> > > +	int reserve = -1;
> > >  
> > >  	/*
> > >  	 * Be lazy and only check for valid flags here,  keeping it out of the
> > > @@ -2816,7 +2816,8 @@ static int cache_grow(struct kmem_cache
> > >  	if (local_flags & __GFP_WAIT)
> > >  		local_irq_disable();
> > >  	check_irq_off();
> > > -	slab_set_reserve(cachep, reserve);
> > > +	if (reserve != -1)
> > > +		slab_set_reserve(cachep, reserve);
> > >  	spin_lock(&l3->list_lock);
> > >  
> > >  	/* Make slab active. */
> > 
> > Given the patch description, shouldn't this be a test for objp != NULL 
> > instead, then?
> 
> In between those to patch hunks, cache_grow contains the code:
> 	if (!objp)
> 		objp = kmem_getpages(cachep, local_flags, nodeid, &reserve);
> 	if (!objp)
> 		goto failed;
> 
> We can no longer test if objp was NULL on entry to the function.
> We could take a copy of objp on entry to the function, and test it
> here.  But initialising 'reserve' to an invalid value is easier.
> 

Seems like you could do all this in kmem_getpages(), then, by calling 
slab_set_reserve(cachep, page->reserve) before returning the new page?

 [ I'd also drop the branch in slab_set_reserve(), it's faster to just 
   assign it unconditionally. ]
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