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


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