[PATCH 30/31] Fix use of uninitialized variable in cache_grow()

[PATCH 30/31] Fix use of uninitialized variable in cache_grow()

[Suresh Jayaraman]

From: Miklos Szeredi <mszeredi@suse.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@suse.cz>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.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. */

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

[David Rientjes]

On Thu, 1 Oct 2009, Suresh Jayaraman wrote:

> From: Miklos Szeredi <mszeredi@suse.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@suse.cz>
> Signed-off-by: Suresh Jayaraman <sjayaraman@suse.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)".

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

[Neil Brown]

On Thursday October 1, rientjes@google.com wrote:
> On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
> 
> > From: Miklos Szeredi <mszeredi@suse.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@suse.cz>
> > Signed-off-by: Suresh Jayaraman <sjayaraman@suse.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?

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.



> 
> 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)".

That change is only appropriate when the compiler is issuing a
warning that the variable is used before it is initialised, but we
know that not to be the case.
In this situation, we know it *is* being used before it is
initialised, and so we need to initialise it to something.

Thanks,
NeilBrown

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

[David Rientjes]

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. ]