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!
