Re: [PATCH 1/1] mm: kfence: skip kmemleak alloc in kfence_pool

[Marco Elver <elver@google.com>]

On Fri, 24 Jun 2022 at 10:20, 'Yee Lee' via kasan-dev
<kasan-dev@googlegroups.com> wrote:
>
> On Thu, 2022-06-23 at 13:59 +0200, Marco Elver wrote:
> > On Thu, 23 Jun 2022 at 13:20, yee.lee via kasan-dev
> > <kasan-dev@googlegroups.com> wrote:
> > >
> > > From: Yee Lee <yee.lee@mediatek.com>
> > >
> > > Use MEMBLOCK_ALLOC_NOLEAKTRACE to skip kmemleak registration when
> > > the kfence pool is allocated from memblock. And the kmemleak_free
> > > later can be removed too.
> >
> > Is this purely meant to be a cleanup and non-functional change?
> >
> > > Signed-off-by: Yee Lee <yee.lee@mediatek.com>
> > >
> > > ---
> > >  mm/kfence/core.c | 18 ++++++++----------
> > >  1 file changed, 8 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> > > index 4e7cd4c8e687..0d33d83f5244 100644
> > > --- a/mm/kfence/core.c
> > > +++ b/mm/kfence/core.c
> > > @@ -600,14 +600,6 @@ static unsigned long kfence_init_pool(void)
> > >                 addr += 2 * PAGE_SIZE;
> > >         }
> > >
> > > -       /*
> > > -        * The pool is live and will never be deallocated from this
> > > point on.
> > > -        * Remove the pool object from the kmemleak object tree, as
> > > it would
> > > -        * otherwise overlap with allocations returned by
> > > kfence_alloc(), which
> > > -        * are registered with kmemleak through the slab post-alloc
> > > hook.
> > > -        */
> > > -       kmemleak_free(__kfence_pool);
> >
> > This appears to only be a non-functional change if the pool is
> > allocated early. If the pool is allocated late using page-alloc, then
> > there'll not be a kmemleak_free() on that memory and we'll have the
> > same problem.
>
> Do you mean the kzalloc(slab_is_available) in memblock_allc()? That
> implies that MEMBLOCK_ALLOC_NOLEAKTRACE has no guarantee skipping
> kmemleak_alloc from this. (Maybe add it?)

No, if KFENCE is initialized through kfence_init_late() ->
kfence_init_pool_late() -> kfence_init_pool().