Messages in this thread | | | From | Neil Brown <> | Date | Fri, 2 Oct 2009 14:54:21 +1000 | Subject | Re: [PATCH 30/31] Fix use of uninitialized variable in cache_grow() |
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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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