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

[Andrew Morton]

On Mon, 2 Jul 2018 13:22:23 -0700 Evgenii Stepanov <eugenis@xxxxxxxxxx> wrote:

> On Mon, Jul 2, 2018 at 12:21 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Mon, 2 Jul 2018 12:16:42 -0700 Evgenii Stepanov <eugenis@xxxxxxxxxx> wrote:
> >
> >> On Fri, Jun 29, 2018 at 7:41 PM, Andrew Morton
> >> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >> > On Fri, 29 Jun 2018 14:45:08 +0200 Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> >> >
> >> >> >> 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.
> >> >>
> >> >> If you think it makes sense, I can also make separate measurements
> >> >> with some workload. What kind of workload should I use?
> >> >
> >> > Whatever workload people were running when they encountered problems
> >> > with KASAN memory consumption ;)
> >> >
> >> > I dunno, something simple.  `find / > /dev/null'?
> >> >
> >>
> >> Looking at a live Android device under load, slab (according to
> >> /proc/meminfo) + kernel stack take 8-10% available RAM (~350MB).
> >> Kasan's overhead of 2x - 3x on top of it is not insignificant.
> >>
> >
> > (top-posting repaired.  Please don't)
> >
> > For a debugging, not-for-production-use feature, that overhead sounds
> > quite acceptable to me.  What problems is it known to cause?
> 
> Not having this overhead enables near-production use - ex. running
> kasan/khasan kernel on a personal, daily-use device to catch bugs that
> do not reproduce in test configuration. These are the ones that often
> cost the most engineering time to track down.
> 
> CPU overhead is bad, but generally tolerable. RAM is critical, in our
> experience. Once it gets low enough, OOM-killer makes your life
> miserable.

OK, anecdotal experience works for me.  But this is all stuff that
should have been in the changelog from day zero, please.  It describes
the reason for the patchset's existence!