Re: BUG: soft lockup in __kmalloc_node() with KFENCE enabled

[Marco Elver <elver@google.com>]

On Mon, 11 Oct 2021 at 08:32, Andrea Righi <andrea.righi@canonical.com> wrote:
> On Mon, Oct 11, 2021 at 08:00:00AM +0200, Marco Elver wrote:
> > On Sun, 10 Oct 2021 at 15:53, Andrea Righi <andrea.righi@canonical.com> wrote:
> > > I can systematically reproduce the following soft lockup w/ the latest
> > > 5.15-rc4 kernel (and all the 5.14, 5.13 and 5.12 kernels that I've
> > > tested so far).
> > >
> > > I've found this issue by running systemd autopkgtest (I'm using the
> > > latest systemd in Ubuntu - 248.3-1ubuntu7 - but it should happen with
> > > any recent version of systemd).
> > >
> > > I'm running this test inside a local KVM instance and apparently systemd
> > > is starting up its own KVM instances to run its tests, so the context is
> > > a nested KVM scenario (even if I don't think the nested KVM part really
> > > matters).
> > >
> > > Here's the oops:
> > >
> > > [   36.466565] watchdog: BUG: soft lockup - CPU#0 stuck for 26s! [udevadm:333]
> > > [   36.466565] Modules linked in: btrfs blake2b_generic zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear psmouse floppy
> > > [   36.466565] CPU: 0 PID: 333 Comm: udevadm Not tainted 5.15-rc4
> > > [   36.466565] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> > [...]
> > >
> > > If I disable CONFIG_KFENCE the soft lockup doesn't happen and systemd
> > > autotest completes just fine.
> > >
> > > We've decided to disable KFENCE in the latest Ubuntu Impish kernel
> > > (5.13) for now, because of this issue, but I'm still investigating
> > > trying to better understand the problem.
> > >
> > > Any hint / suggestion?
> >
> > Can you confirm this is not a QEMU TCG instance? There's been a known
> > issue with it: https://bugs.launchpad.net/qemu/+bug/1920934
>
> It looks like systemd is running qemu-system-x86 without any "accel"
> options, so IIUC the instance shouldn't use TCG. Is this a correct
> assumption or is there a better way to check?

AFAIK, the default is TCG if nothing else is requested. What was the
command line?

> > One thing that I've been wondering is, if we can make
> > CONFIG_KFENCE_STATIC_KEYS=n the default, because the static keys
> > approach is becoming more trouble than it's worth. It requires us to
> > re-benchmark the defaults. If you're thinking of turning KFENCE on by
> > default (i.e. CONFIG_KFENCE_SAMPLE_INTERVAL non-zero), you could make
> > this decision for Ubuntu with whatever sample interval you choose.
> > We've found that for large deployments 500ms or above is more than
> > adequate.
>
> Another thing that I forgot to mention is that with
> CONFIG_KFENCE_STATIC_KEYS=n the soft lockup doesn't seem to happen.

Thanks for confirming.

Thanks,
-- Marco