Re: [PATCH 2/2] mm: kfence: fix objcgs vector allocation

[Marco Elver <elver@google.com>]

On Mon, 28 Mar 2022 at 03:53, Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Mon, Mar 28, 2022 at 1:31 AM Marco Elver <elver@google.com> wrote:
> >
> > On Sun, 27 Mar 2022 at 07:19, Muchun Song <songmuchun@bytedance.com> wrote:
> > >
> > > If the kfence object is allocated to be used for objects vector, then
> > > this slot of the pool eventually being occupied permanently since
> > > the vector is never freed.  The solutions could be 1) freeing vector
> > > when the kfence object is freed or 2) allocating all vectors statically.
> > > Since the memory consumption of object vectors is low, it is better to
> > > chose 2) to fix the issue and it is also can reduce overhead of vectors
> > > allocating in the future.
> > >
> > > Fixes: d3fb45f370d9 ("mm, kfence: insert KFENCE hooks for SLAB")
> > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > ---
> > >  mm/kfence/core.c   | 3 +++
> > >  mm/kfence/kfence.h | 1 +
> > >  2 files changed, 4 insertions(+)
> >
> > Thanks for this -- mostly looks good. Minor comments below + also
> > please fix what the test robot reported.
>
> Will do.
>
> >
> > > diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> > > index 13128fa13062..9976b3f0d097 100644
> > > --- a/mm/kfence/core.c
> > > +++ b/mm/kfence/core.c
> > > @@ -579,9 +579,11 @@ static bool __init kfence_init_pool(void)
> > >         }
> > >
> > >         for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> > > +               struct slab *slab = virt_to_slab(addr);
> > >                 struct kfence_metadata *meta = &kfence_metadata[i];
> > >
> > >                 /* Initialize metadata. */
> > > +               slab->memcg_data = (unsigned long)&meta->objcg | MEMCG_DATA_OBJCGS;
> >
> > Maybe just move it to kfence_guarded_alloc(), see "/* Set required
> > slab fields */", where similar initialization on slab is done.
>
> But slab->memcg_data is special since it is only needed to be
> initialized once.  I think it is better move it to the place where
> __SetPageSlab(&pages[i]) is.  What do you think?

That's fair.