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

[Nicolin Chen]

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.....

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/mm/hmm.c#n1144

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
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? 
 
> 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?

Thanks!