Re: [PATCH] arm64: mte: Use PAGE_KERNEL_TAGGED in arch_add_memory

[David Hildenbrand <david@redhat.com>]

On 05.03.21 16:43, Catalin Marinas wrote:
> On Fri, Mar 05, 2021 at 11:19:08AM +0000, Catalin Marinas wrote:
>> On Tue, Mar 02, 2021 at 08:21:03PM -0800, Patrick Daly wrote:
>>> 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 <pdaly@codeaurora.org>
>>> ---
>>>   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.
> 
> I played with this a bit and the last option is not really feasible.
> There are several places in the kernel where PAGE_KERNEL is used just
> because the expectation is for write-back memory. For MTE, we need
> tagged memory either because the kernel allocators need it (with
> KASAN_HW_TAGS) or because it is shared with the user, potentially mapped
> as tagged in user space and such tags need to be preserved by the
> kernel. So for the latter, only the linear map needs to be tagged (and
> memory hotplug falls into this category).
> 
> This leaves us with one of the first two options. I think the easiest
> is:
> 
> ---------------8<-------------------------------
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e17b96d0e4b5..5c78b92d9ec5 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -486,6 +486,8 @@ static inline pmd_t pmd_mkdevmap(pmd_t pmd)
>   	__pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_NORMAL_NC) | PTE_PXN | PTE_UXN)
>   #define pgprot_device(prot) \
>   	__pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_DEVICE_nGnRE) | PTE_PXN | PTE_UXN)
> +#define pgprot_tagged(prot) \
> +	__pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_NORMAL_TAGGED))
>   /*
>    * DMA allocations for non-coherent devices use what the Arm architecture calls
>    * "Normal non-cacheable" memory, which permits speculation, unaligned accesses
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index cdfc4e9f253e..f5f5044db2ce 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -904,6 +904,10 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
>   #define pgprot_device pgprot_noncached
>   #endif
> 
> +#ifndef pgprot_tagged
> +#define pgprot_tagged(prot)	(prot)
> +#endif
> +
>   #ifdef CONFIG_MMU
>   #ifndef pgprot_modify
>   #define pgprot_modify pgprot_modify
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 5ba51a8bdaeb..4253d80a59ba 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1072,7 +1072,7 @@ static int online_memory_block(struct memory_block *mem, void *arg)
>    */
>   int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
>   {
> -	struct mhp_params params = { .pgprot = PAGE_KERNEL };
> +	struct mhp_params params = { .pgprot = pgprot_tagged(PAGE_KERNEL) };

This looks like we're pushing arch specific stuff ("tagged") in here. 
Can't we generalize this to something like

pgprot_mhp_default

(or a better name)

that defaults to PAGE_KERNEL on all architectures except arm64 which 
overwrites this somehow?


-- 
Thanks,

David / dhildenb