Re: [PATCH v4 00/17] khwasan: kernel hardware assisted address sanitizer

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Jun 29, 2018 at 02:45:08PM +0200, Andrey Konovalov wrote:
> On Thu, Jun 28, 2018 at 9:40 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Thu, 28 Jun 2018 20:29:07 +0200 Andrey Konovalov <andreyknvl@google.com> wrote:
> >
> >> >> Slab memory usage after boot [2]:
> >> >> * ~40 kb for clean kernel
> >> >> * ~105 kb + 1/8th shadow ~= 118 kb for KASAN
> >> >> * ~47 kb + 1/16th shadow ~= 50 kb for KHWASAN
> >> >>
> >> >> Network performance [3]:
> >> >> * 8.33 Gbits/sec for clean kernel
> >> >> * 3.17 Gbits/sec for KASAN
> >> >> * 2.85 Gbits/sec for KHWASAN
> >> >>
> >> >> Note, that KHWASAN (compared to KASAN) doesn't require quarantine.
> >> >>
> >> >> [1] Time before the ext4 driver is initialized.
> >> >> [2] Measured as `cat /proc/meminfo | grep Slab`.
> >> >> [3] Measured as `iperf -s & iperf -c 127.0.0.1 -t 30`.
> >> >
> >> > The above doesn't actually demonstrate the whole point of the
> >> > patchset: to reduce KASAN's very high memory consumption?
> >>
> >> You mean that memory usage numbers collected after boot don't give a
> >> representative picture of actual memory consumption on real workloads?
> >>
> >> What kind of memory consumption testing would you like to see?
> >
> > Well, 100kb or so is a teeny amount on virtually any machine.  I'm
> > assuming the savings are (much) more significant once the machine gets
> > loaded up and doing work?
> 
> So with clean kernel after boot we get 40 kb memory usage. With KASAN
> it is ~120 kb, which is 200% overhead. With KHWASAN it's 50 kb, which
> is 25% overhead. This should approximately scale to any amounts of
> used slab memory. For example with 100 mb memory usage we would get
> +200 mb for KASAN and +25 mb with KHWASAN. (And KASAN also requires
> quarantine for better use-after-free detection). I can explicitly
> mention the overhead in %s in the changelog.

Could you elaborate on where that SLAB overhead comes from?

IIUC that's not for the shadow itself (since it's allocated up-front and
not accounted to SLAB), and that doesn't take into account the
quarantine, so what's eating that space?

Thanks,
Mark.