Re: [PATCH 30/31] Fix use of uninitialized variable incache_grow()

From: David Rientjes
Date: Thu Oct 01 2009 - 16:49:23 EST


On Thu, 1 Oct 2009, Suresh Jayaraman wrote:

> From: Miklos Szeredi <mszeredi@xxxxxxx>
>
> 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@xxxxxxx>
> Signed-off-by: Suresh Jayaraman <sjayaraman@xxxxxxx>
> ---
> 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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