On Wed, Jan 04, 2017 at 01:50:20PM +0000, Ard Biesheuvel wrote:

On 4 January 2017 at 13:28, Will Deacon [off-list ref] wrote:

quoted

On Wed, Dec 14, 2016 at 09:11:47AM +0000, Ard Biesheuvel wrote:

quoted

The NUMA code may get confused by the presence of NOMAP regions within
zones, resulting in spurious BUG() checks where the node id deviates
from the containing zone's node id.

Since the kernel has no business reasoning about node ids of pages it
does not own in the first place, enable CONFIG_HOLES_IN_ZONE to ensure
that such pages are disregarded.

Signed-off-by: Ard Biesheuvel <redacted>
---
 arch/arm64/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 111742126897..0472afe64d55 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -614,6 +614,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
      def_bool y
      depends on NUMA

+config HOLES_IN_ZONE
+     def_bool y
+     depends on NUMA
+
 source kernel/Kconfig.preempt
 source kernel/Kconfig.hz

I'm happy to apply this, but I'll hold off until the first patch is queued
somewhere, since this doesn't help without the VM_BUG_ON being moved.

Alternatively, I can queue both if somebody from the mm camp acks the
first patch.

Actually, I am not convinced the discussion is finalized. These
patches do fix the issue, but Robert also suggested an alternative fix
which may be preferable.

http://marc.info/?l=linux-arm-kernel&m=148190753510107&w=2

I haven't responded to it yet, due to the holidays, but I'd like to
explore that solution a bit further before applying anything, if you
don't mind.

Using early_pfn_valid feels like a bodge to me, since having pfn_valid
return false for something that early_pfn_valid says is valid (and is
therefore initialised in the memmap) makes the NOMAP semantics even more
confusing.

But there's no rush, so I'll hold off for the moment. I was under the
impression that things had stalled.

Will