Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>,
	Marco Elver <elver@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christoph Lameter <cl@linux.com>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Pekka Enberg <penberg@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eric Dumazet <edumazet@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Peter Zijlstra <peterz@infradead.org>, Qian Cai <cai@lca.pw>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will@kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure
Date: Thu, 10 Sep 2020 13:25:26 -0700	[thread overview]
Message-ID: <20200910202526.GU29330@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <CACT4Y+awrz-j8y5Qc8OS9qkov4doMnw1V=obwp3MB_LTvaUFXw@mail.gmail.com>

On Thu, Sep 10, 2020 at 07:11:41PM +0200, Dmitry Vyukov wrote:
> On Thu, Sep 10, 2020 at 6:19 PM Alexander Potapenko <glider@google.com> wrote:
> >
> > On Thu, Sep 10, 2020 at 5:43 PM Dmitry Vyukov <dvyukov@google.com> wrote:
> >
> >
> > > > +       /* Calculate address for this allocation. */
> > > > +       if (right)
> > > > +               meta->addr += PAGE_SIZE - size;
> > > > +       meta->addr = ALIGN_DOWN(meta->addr, cache->align);
> > >
> > > I would move this ALIGN_DOWN under the (right) if.
> > > Do I understand it correctly that it will work, but we expect it to do
> > > nothing for !right? If cache align is >PAGE_SIZE, nothing good will
> > > happen anyway, right?
> > > The previous 2 lines look like part of the same calculation -- "figure
> > > out the addr for the right case".
> >
> > Yes, makes sense.
> >
> > > > +
> > > > +       schedule_delayed_work(&kfence_timer, 0);
> > > > +       WRITE_ONCE(kfence_enabled, true);
> > >
> > > Can toggle_allocation_gate run before we set kfence_enabled? If yes,
> > > it can break. If not, it's still somewhat confusing.
> >
> > Correct, it should go after we enable KFENCE. We'll fix that in v2.
> >
> > > > +void __kfence_free(void *addr)
> > > > +{
> > > > +       struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> > > > +
> > > > +       if (unlikely(meta->cache->flags & SLAB_TYPESAFE_BY_RCU))
> > >
> > > This may deserve a comment as to why we apply rcu on object level
> > > whereas SLAB_TYPESAFE_BY_RCU means slab level only.
> >
> > Sorry, what do you mean by "slab level"?
> > SLAB_TYPESAFE_BY_RCU means we have to wait for possible RCU accesses
> > in flight before freeing objects from that slab - that's basically
> > what we are doing here below:
> 
> Exactly! You see it is confusing :)
> SLAB_TYPESAFE_BY_RCU does not mean that. rcu-freeing only applies to
> whole pages, that's what I mean by "slab level" (whole slabs are freed
> by rcu).

Just confirming Dmitry's description of SLAB_TYPESAFE_BY_RCU semantics.

							Thanx, Paul