Kernel/driver hacking: panic: Assertion vm_object_busied((m->object)) failed at /usr/src/sys/vm/vm_page.c:5455

Hi current@,

First off, sorry if I spammed developers@ and other mailing lists with 
my previous message, and to bz@/hselasky@/manu@ sent so many duplicate 
emails.

Right now, I am attempting to update the drm-kmod driver to the Linux 
5.7 code, and am having an issue with the pmap lock. I am new-ish to the 
kernel, meaning not a whole lot of "experience", but do have patches in 
src.

But like it not we need kernel newbies, they're the next generation of 
experts. If we don't, we'd be the next Minix with **zero** development 
since Tanenbaum retired.

Going back, the code in question is here: 
https://github.com/neelchauhan/drm-kmod/blob/5.7-wip/drivers/gpu/drm/i915/gem/i915_gem_mman.c#L346

The lines important are 346-356, but lines of interest are also the 
non-"#ifdef __linux__" sections of vm_fault_cpu().

The code gives this error: panic: Assertion 
vm_object_busied((m->object)) failed at /usr/src/sys/vm/vm_page.c:5455

I have attached the core dump log.

To those who aren't graphics driver experts, it happens when I load Xorg 
when Xorg attempts to map the I/O to userspace. But I feel this is more 
of me not using page locks correctly (which is needed for the pmap), or 
maybe a linuxkpi issue, rather than a graphics-specific issue.

I spent days on this (all my non-$DAYJOB hours at one point + all my 
weekends) and haven't figured out the locks completely. Does anyone have 
suggestions to what I'm doing wrong in my code and locks?

If it is important, OpenBSD's version of this code is here: 
https://github.com/openbsd/src/blob/2207c4325726fdc5c4bcd0011af0fdf7d3dab137/sys/dev/pci/drm/i915/gem/i915_gem_mman.c#L459 
(lines 459-523, but some calls are unsurprisingly different).

Hope you all can help.

-Neel (nc@)