Re: [RFC][PATCH] arm64: update iomem_resource.end

From: Robin Murphy
Date: Fri May 11 2018 - 07:12:42 EST

On 10/05/18 23:29, Nicolin Chen wrote:
Thanks for the comments, Robin.

On Thu, May 10, 2018 at 06:45:59PM +0100, Robin Murphy wrote:
On 09/05/18 23:58, Nicolin Chen wrote:
The iomem_resource.end is -1 by default and should be updated in
arch-level code.

ARM64 so far hasn't updated it while core kernel code (mm/hmm.c)
started to use iomem_resource.end for boundary check. So it'd be
better to assign iomem_resource.end using a valid value, the end
of physical address space for example because iomem_resource.end
in theory should reflect that.

However, VA_BITS might be smaller than PA_BITS in ARM64. So using
the end of physical address space doesn't make a lot of sense in
this case, or could be even harmful since virtual address cannot
reach that memory region.

Why? There's plenty of stuff in the physical address space that will
only ever be accessed via ioremap/memremap. There's no reason you
shouldn't be able to run a VA_BITS < 48 kernel on a Cavium ThunderX

I'm running VA_BITS_39 and PA_BITS_48 on Tegra 210. There had
not been any problem of it, however with hmm.....

This hmm_devmem_add() requests a region with PFNs being outside
of the linear region in ARM64 case which takes MAX_PHYSMEM_BITS
(48 bits) over iomem_resource.end without this patch. Then when
dealing with page structures in vmemmap region from a given PFN
directly (CONFIG_SPARSEMEM_VMEMMAP=y), and the given PFN is the
last one based on physical region (48 bits), the address of its
page structure will go beyond vmemmap region. Does this sound a

Yes, but as far as we're concerned here it's not a problem with arm64:

depends on (X86_64 || PPC64)
depends on ZONE_DEVICE

Whatever out-of-tree changes you have to address all of those are clearly implemented incorrectly; *that's* your problem.

where *all* the I/O is in the top half of the PA space. We already
constrain RAM in this very function to those regions which fit into
the linear map, and if you're accessing anything other than RAM
through the linear map you're probably doing something wrong.

If I understand this part correctly, since ARM64 has applied the
memory limit already, does it mean that probably we should fix
something in the region_intersects() or add an extra check in the
hmm_devmem_add(), instead of limiting the iomem_resource?

It means we should implement memory hotplug correctly. Which, unfortunately, I happen to know is really hard (it's something I've been looking at from the device-DAX angle).

Furthermore, the physical region covered by the linear map doesn't
necessarily start at physical address 0 anyway - see PHYS_OFFSET.

Hmm...okay...but there still should be a protection somewhere if
it happens to access a page structure via pfn_to_page() while the
PFN is not covered by the vmemmap linear mapping, right?

There already is: pfn_valid() will return false for anything outside the intersection of memblock regions and the linear map region as calculated by arm64_memblock_init(); anything calling pfn_to_page() without checking pfn_valid() first is fundamentally broken. Or if you have out-of-tree changes to the pfn_valid() implementation then all bets are off, and it's not something for mainline to work around.