Re: [RFC] arm64: mm: update max_pfn after memory hotplug
From: Florian Fainelli <f.fainelli@gmail.com>
Date: 2021-09-24 02:47:18
Also in:
linux-mm, lkml
On 9/23/2021 3:54 PM, Chris Goldsworthy wrote:
quoted hunk ↗ jump to hunk
From: Sudarshan Rajagopalan <redacted> After new memory blocks have been hotplugged, max_pfn and max_low_pfn needs updating to reflect on new PFNs being hot added to system. Signed-off-by: Sudarshan Rajagopalan <redacted> Signed-off-by: Chris Goldsworthy <redacted> --- arch/arm64/mm/mmu.c | 5 +++++ 1 file changed, 5 insertions(+)diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index cfd9deb..fd85b51 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c@@ -1499,6 +1499,11 @@ int arch_add_memory(int nid, u64 start, u64 size, if (ret) __remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), size); + else { + max_pfn = PFN_UP(start + size); + max_low_pfn = max_pfn; + }
This is a drive by review, but it got me thinking about your changes a bit: - if you raise max_pfn when you hotplug memory, don't you need to lower it when you hot unplug memory as well? - suppose that you have a platform which maps physical memory into the CPU's address space at 0x00_4000_0000 (1GB offset) and the kernel boots with 2GB of DRAM plugged by default. At that point we have not registered a swiotlb because we have less than 4GB of addressable physical memory, there is no IOMMU in that system, it's a happy world. Now assume that we plug an additional 2GB of DRAM into that system adjacent to the previous 2GB, from 0x00_C0000_0000 through 0x14_0000_0000, now we have physical addresses above 4GB, but we still don't have a swiotlb, some of our DMA_BIT_MASK(32) peripherals are going to be unable to DMA from that hot plugged memory, but they could if we had a swiotlb. - now let's go even further but this is very contrived. Assume that the firmware has somewhat created a reserved memory region with a 'no-map' attribute thus indicating it does not want a struct page to be created for a specific PFN range, is it valid to "blindly" raise max_pfn if that region were to be at the end of the just hot-plugged memory? -- Florian _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel