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

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Date:   Mon, 28 Mar 2022 09:01:55 +0200
From:   Marco Elver <elver@...gle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: kfence: fix objcgs vector allocation

On Mon, 28 Mar 2022 at 03:53, Muchun Song <songmuchun@...edance.com> wrote:
>
> On Mon, Mar 28, 2022 at 1:31 AM Marco Elver <elver@...gle.com> wrote:
> >
> > On Sun, 27 Mar 2022 at 07:19, Muchun Song <songmuchun@...edance.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@...edance.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.