Messages in this thread | | | Subject | Re: [RFC][PATCH] arm64: update iomem_resource.end | From | Robin Murphy <> | Date | Fri, 11 May 2018 12:12:32 +0100 |
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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..... > > 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?
Yes, but as far as we're concerned here it's not a problem with arm64:
config ARCH_HAS_HMM ... depends on (X86_64 || PPC64) depends on ZONE_DEVICE ... depends on MEMORY_HOTPLUG depends on MEMORY_HOTREMOVE ...
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.
Robin.
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