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Date: Sat, 22 Jul 2017 11:00:12 +0300
From: Konstantin Belousov <kostikbel@gmail.com>
To: Eugene Grosbein <eugen@grosbein.net>
Cc: Don Lewis <truckman@FreeBSD.org>, pz-freebsd-stable@ziemba.us,
 freebsd-stable@FreeBSD.org
Subject: Re: stable/11 r321349 crashing immediately
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On Sat, Jul 22, 2017 at 02:40:59PM +0700, Eugene Grosbein wrote:
> Also, I've always wondered what load pattern one should have
> to exhibit real kernel stack problems due to KVA fragmentation
> and KSTACK_PAGES>2 on i386?
In fact each stack consumes 3 contigous pages because there is also
the guard, which catches the double faults.

You need to use the machine, e.g. to run something that creates and destroys
kernel threads, while doing something that consumes kernel_arena KVA.
Plain malloc/zmalloc is enough.

In other words, any non-static load would cause fragmentation preventing
allocations of the kernel stacks for new threads.

> How can I get ddb backtrace you asked for? I'm not very familiar with ddb.
> I have serial console to such i386 system.

bt command for the given thread provides the backtrace.  I have no idea
how did you obtained the numbers that you show.