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

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Message-ID: <alpine.DEB.1.00.0910011341280.27559@chino.kir.corp.google.com>
Date:	Thu, 1 Oct 2009 13:49:00 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Suresh Jayaraman <sjayaraman@...e.de>
cc:	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, Neil Brown <neilb@...e.de>,
	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 Thu, 1 Oct 2009, Suresh Jayaraman wrote:

> From: Miklos Szeredi <mszeredi@...e.cz>
> 
> This fixes a bug in reserve-slub.patch.
> 
> If cache_grow() was called with objp != NULL then the 'reserve' local
> variable wasn't initialized. This resulted in ac->reserve being set to
> a rubbish value.  Due to this in some circumstances huge amounts of
> slab pages were allocated (due to slab_force_alloc() returning true),
> which caused atomic page allocation failures and slowdown of the
> system.
> 
> Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
> Signed-off-by: Suresh Jayaraman <sjayaraman@...e.de>
> ---
>  mm/slab.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> 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?

If so, it doesn't make sense because reserve will only be initialized when 
objp == NULL in the call to kmem_getpages() from cache_grow().


The title of the patch suggests this is just dealing with an uninitialized 
auto variable so the anticipated change would be from "int reserve" to 
"int uninitialized_var(result)".
--
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