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

[Andrea Arcangeli]

Hello,

On Tue, Sep 27, 2016 at 05:16:15AM -0500, Shaun Tancheff wrote:
> git bisect points at commit  c9634dcf00c9c93b ("mm: vma_merge: fix
> vm_page_prot SMP race condition against rmap_walk")

I assume linux-next? But I can't find the commit, but I should know
what this is.

> 
> 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)

It's a false positive, you have DEBUG_VM_RB=y, you can disable it or
cherry-pick the fix:

https://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/commit/?id=74d8b44224f31153e23ca8a7f7f0700091f5a9b2

The assumption validate_mm_rb did isn't valid anymore on the new code
during __vma_unlink, the validation code must be updated to skip the
next vma instead of the current one after this change. It's a bug in
DEBUG_VM_RB=y, if you keep DEBUG_VM_RB=n there's no bug.

> 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.

I already submitted the fix to Andrew last week:

https://marc.info/?l=linux-mm&m=147449253801920&w=2

I assume it's pending for merging in -mm.

If you can test this patch and confirm the problem goes away with
DEBUG_VM_RB=y it'd be great.

Thanks,
Andrea