BUG Re: mm: vma_merge: fix vm_page_prot SMP race condition against rmap_walk

[Shaun Tancheff]

git bisect points at commit  c9634dcf00c9c93b ("mm: vma_merge: fix
vm_page_prot SMP race condition against rmap_walk")

Last lines to console are [transcribed]:

vma ffff8c3d989a7c78 start 00007fe02ed4c000 end 00007fe02ed52000
next ffff8c3d96de0c38 prev ffff8c3d989a6e40 mm ffff8c3d071cbac0
prot 8000000000000025 anon_vma ffff8c3d96fc9b28 vm_ops           (null)
pgoff 7fe02ed4c file           (null) private_data           (null)
flags: 0x8100073(read|write|mayread|maywrite|mayexec|account|softdirty)

Reproducer is an Ubuntu 16.04.1 LTS x86_64 running on a VM (VirtualBox).
Symptom is a solid hang after boot and switch to starting gnome session.

Hang at about 35s.

kdbg traceback is all null entries.

Let me know what additional information I can provide.

Regards,
Shaun Tancheff