Re: [PATCH RFC 00/10] KFENCE: A low-overhead sampling-based memory safety error detector

From: Dmitry Vyukov
Date: Fri Sep 11 2020 - 03:36:22 EST

On Tue, Sep 8, 2020 at 5:56 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> On Tue, Sep 08, 2020 at 05:36PM +0200, Vlastimil Babka wrote:
> > On 9/8/20 5:31 PM, Marco Elver wrote:
> > >>
> > >> How much memory overhead does this end up having? I know it depends on
> > >> the object size and so forth. But, could you give some real-world
> > >> examples of memory consumption? Also, what's the worst case? Say I
> > >> have a ton of worst-case-sized (32b) slab objects. Will I notice?
> > >
> > > KFENCE objects are limited (default 255). If we exhaust KFENCE's memory
> > > pool, no more KFENCE allocations will occur.
> > > Documentation/dev-tools/kfence.rst gives a formula to calculate the
> > > KFENCE pool size:
> > >
> > > The total memory dedicated to the KFENCE memory pool can be computed as::
> > >
> > > ( #objects + 1 ) * 2 * PAGE_SIZE
> > >
> > > Using the default config, and assuming a page size of 4 KiB, results in
> > > dedicating 2 MiB to the KFENCE memory pool.
> > >
> > > Does that clarify this point? Or anything else that could help clarify
> > > this?
> >
> > Hmm did you observe that with this limit, a long-running system would eventually
> > converge to KFENCE memory pool being filled with long-aged objects, so there
> > would be no space to sample new ones?
> Sure, that's a possibility. But remember that we're not trying to
> deterministically detect bugs on 1 system (if you wanted that, you
> should use KASAN), but a fleet of machines! The non-determinism of which
> allocations will end up in KFENCE, will ensure we won't end up with a
> fleet of machines of identical allocations. That's exactly what we're
> after. Even if we eventually exhaust the pool, you'll still detect bugs
> if there are any.
> If you are overly worried, either the sample interval or number of
> available objects needs to be tweaked to be larger. The default of 255
> is quite conservative, and even using something larger on a modern
> system is hardly noticeable. Choosing a sample interval & number of
> objects should also factor in how many machines you plan to deploy this
> on. Monitoring /sys/kernel/debug/kfence/stats can help you here.

Hi Marco,

I reviewed patches and they look good to me (minus some local comments
that I've left).

The main question/concern I have is what Vlastimil mentioned re
long-aged objects.
Is the default sample interval values reasonable for typical
workloads? Do we have any guidelines on choosing the sample interval?
Should it depend on workload/use pattern?
By "reasonable" I mean if the pool will last long enough to still
sample something after hours/days? Have you tried any experiments with
some workload (both short-lived processes and long-lived
processes/namespaces) capturing state of the pool? It can make sense
to do to better understand dynamics. I suspect that the rate may need
to be orders of magnitude lower.

Also I am wondering about the boot process (both kernel and init).
It's both inherently almost the same for the whole population of
machines and inherently produces persistent objects. Should we lower
the rate for the first minute of uptime? Or maybe make it proportional
to uptime?

I feel it's quite an important aspect. We can have this awesome idea
and implementation, but radically lower its utility by using bad
sampling value (which will have silent "failure mode" -- no bugs

But to make it clear: all of this does not conflict with the merge of
the first version. Just having tunable sampling interval is good
enough. We will get the ultimate understanding only when we start
using it widely anyway.