Regular oops on shutdown of KVM/ARM64 machines with VGA device

[Steve Capper]

On 1 July 2015 at 12:27, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Wed, Jul 01, 2015 at 09:20:28AM +0100, Marc Zyngier wrote:
>> [+Will, Catalin]
>>
>> On 30/06/15 19:50, Christoffer Dall wrote:
>> > On Tue, Jun 30, 2015 at 05:20:11PM +0100, Marc Zyngier wrote:
>> >> On 30/06/15 17:16, Dirk Müller wrote:
>> >>> Hi Marc,
>> >>>
>> >>>> Can try the following patch?
>> >>>
>> >>> [..]
>> >>>
>> >>> Thanks a lot for the quick patch, from a brief testing this seems to
>> >>> fix the issue (on a 4k kernel). I'll retest this in our original
>> >>> configuration (which was 64k) but so far I don't see a reason why it
>> >>> shouldn't fix the issue.
>> >>
>> >> Awesome. Mind if I put your Tested-by on the patch?
>> >>
>> > Looks to me like the definition of pmd_huge() on arm64 is broken; pretty
>> > sure when I reviewed this original patch I followed the path of both
>> > pmd_huge() and pmd_trans_huge() and checked that they don't return true
>> > if the entry is clear.  This happens to be the case on both arm and x86,
>> > and I probably only looked at the arm code and not the arm64 code.
>> >
>> > I'm fine with this patch, but I think we should also merge the
>> > following, since by definition, a clear pmd cannot also be a huge pmd:
>> >
>> > diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
>> > index 2de9d2e..779520b 100644
>> > --- a/arch/arm64/mm/hugetlbpage.c
>> > +++ b/arch/arm64/mm/hugetlbpage.c
>> > @@ -40,7 +40,7 @@ int huge_pmd_unshare(struct mm_struct *mm, unsigned long *addr, pte_t *ptep)
>> >
>> >  int pmd_huge(pmd_t pmd)
>> >  {
>> > -   return !(pmd_val(pmd) & PMD_TABLE_BIT);
>> > +   return pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT);
>> >  }
>> >
>> >  int pud_huge(pud_t pud)
>>
>> If the convention is for pmd_huge to check for pmd_none, then we don't
>> need my patch, and only this should be merged.
>
> Adding Steve on cc. I can see that the mm code checks for pmd_none()
> before calling pmd_huge() but I'm not sure it does this all the time
> (same goes for pud_huge).
>
> Steve, do you have any more insight here?
>

I thought pmd_none was always called before pmd_huge, but this was an
oversight on my part as clear pud's and pmd's cannot also be huge.
I think Christoffer's patch should be applied (with the equivalent for
pud_huge too) in case the logic ever changes.

Cheers,
--
Steve