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