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

[Yee Lee <yee.lee@mediatek.com>]

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?)

If so, we cannot identify later the block is stored in the phys
tree(memblock) or the virt tree(page_alloc).