Re: [PATCH] arm64: mte: Use PAGE_KERNEL_TAGGED in arch_add_memory
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2021-03-05 11:21:01
Hi Patrick, Thanks for this. Adding a few more people and linux-mm. On Tue, Mar 02, 2021 at 08:21:03PM -0800, Patrick Daly wrote:
quoted hunk ↗ jump to hunk
In a system which supports MTE, the linear kernel region must allow reading/writing allocation tags. For memory present at boot this is already being done in map_mem(). Add the same in arch_add_memory(). Signed-off-by: Patrick Daly <redacted> --- arch/arm64/mm/mmu.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index 9b25d60b..0fcfe90 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c@@ -1463,6 +1463,7 @@ int arch_add_memory(int nid, u64 start, u64 size, struct mhp_params *params) { int ret, flags = 0; + pgprot_t pgprot; if (!inside_linear_region(start, size)) { pr_err("[%llx %llx] is outside linear mapping region\n", start, start + size);@@ -1477,8 +1478,17 @@ int arch_add_memory(int nid, u64 start, u64 size, IS_ENABLED(CONFIG_KFENCE)) flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; + /* + * The linear map must allow allocation tags reading/writing + * if MTE is present. Otherwise, it has the same attributes as + * PAGE_KERNEL. + */ + pgprot = params->pgprot; + if (pgprot_val(pgprot) == pgprot_val(PAGE_KERNEL)) + pgprot = PAGE_KERNEL_TAGGED; + __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start), - size, params->pgprot, __pgd_pgtable_alloc, + size, pgprot, __pgd_pgtable_alloc, flags);
We'll need a similar pattern for vmalloc() once we have the khwasan support in place. So we could add a pgprot_tagged() function (similar to pgprot_writecombine() etc.) which does the above check and returns PAGE_KERNEL_TAGGED, maybe only checking the PTE_ATTRINDX_MASK bits rather than the whole prot bits. However, the bigger problem is that arch_add_memory() is also called for ZONE_DEVICE mappings and we can't always guarantee that such range supports tagging (most likely it doesn't, e.g. persistent memory), leading to potential external aborts. One option is to expand mhp_params to pass additional information so that the arch code can make the right decision. Another option is to make PAGE_KERNEL_TAGGED global which is just PAGE_KERNEL for all the other architectures and use it in the core code. Yet another option which we haven't fully explored with MTE is to have PAGE_KERNEL always tagged but add a new PAGE_KERNEL_DEVICE (or _UNTAGGED) for specific cases like ZONE_DEVICE. We need to make sure that PAGE_KERNEL doesn't end up in places where the backing memory does not support tags. I'll give the last option a quick try and see if it falls apart (just changing PAGE_KERNEL to tagged). In terms of tag cache usage, it probably won't have much of an impact since the whole of the linear map is tagged already. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel