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

On 1 July 2015 at 12:27, Catalin Marinas [off-list ref] wrote:

On Wed, Jul 01, 2015 at 09:20:28AM +0100, Marc Zyngier wrote:

quoted

[+Will, Catalin]

On 30/06/15 19:50, Christoffer Dall wrote:

quoted

On Tue, Jun 30, 2015 at 05:20:11PM +0100, Marc Zyngier wrote:

quoted

On 30/06/15 17:16, Dirk M?ller wrote:

quoted

Hi Marc,

quoted

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