Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings

[Ryan Roberts <ryan.roberts@arm.com>]

On 13/02/2024 13:13, David Hildenbrand wrote:
> On 13.02.24 14:06, Ryan Roberts wrote:
>> On 13/02/2024 12:19, David Hildenbrand wrote:
>>> On 13.02.24 13:06, Ryan Roberts wrote:
>>>> On 12/02/2024 20:38, Ryan Roberts wrote:
>>>>> [...]
>>>>>
>>>>>>>>> +static inline bool mm_is_user(struct mm_struct *mm)
>>>>>>>>> +{
>>>>>>>>> +    /*
>>>>>>>>> +     * Don't attempt to apply the contig bit to kernel mappings, because
>>>>>>>>> +     * dynamically adding/removing the contig bit can cause page faults.
>>>>>>>>> +     * These racing faults are ok for user space, since they get
>>>>>>>>> serialized
>>>>>>>>> +     * on the PTL. But kernel mappings can't tolerate faults.
>>>>>>>>> +     */
>>>>>>>>> +    return mm != &init_mm;
>>>>>>>>> +}
>>>>>>>>
>>>>>>>> We also have the efi_mm as a non-user mm, though I don't think we
>>>>>>>> manipulate
>>>>>>>> that while it is live, and I'm not sure if that needs any special handling.
>>>>>>>
>>>>>>> Well we never need this function in the hot (order-0 folio) path, so I
>>>>>>> think I
>>>>>>> could add a check for efi_mm here with performance implication. It's
>>>>>>> probably
>>>>>>> safest to explicitly exclude it? What do you think?
>>>>>>
>>>>>> Oops: This should have read "I think I could add a check for efi_mm here
>>>>>> *without* performance implication"
>>>>>
>>>>> It turns out that efi_mm is only defined when CONFIG_EFI is enabled. I can do
>>>>> this:
>>>>>
>>>>> return mm != &init_mm && (!IS_ENABLED(CONFIG_EFI) || mm != &efi_mm);
>>>>>
>>>>> Is that acceptable? This is my preference, but nothing else outside of efi
>>>>> references this symbol currently.
>>>>>
>>>>> Or perhaps I can convince myself that its safe to treat efi_mm like userspace.
>>>>> There are a couple of things that need to be garanteed for it to be safe:
>>>>>
>>>>>     - The PFNs of present ptes either need to have an associated struct
>>>>> page or
>>>>>       need to have the PTE_SPECIAL bit set (either pte_mkspecial() or
>>>>>       pte_mkdevmap())
>>>>>
>>>>>     - Live mappings must either be static (no changes that could cause
>>>>> fold/unfold
>>>>>       while live) or the system must be able to tolerate a temporary fault
>>>>>
>>>>> Mark suggests efi_mm is not manipulated while live, so that meets the latter
>>>>> requirement, but I'm not sure about the former?
>>>>
>>>> I've gone through all the efi code, and conclude that, as Mark suggests, the
>>>> mappings are indeed static. And additionally, the ptes are populated using only
>>>> the _private_ ptep API, so there is no issue here. As just discussed with Mark,
>>>> my prefereence is to not make any changes to code, and just add a comment
>>>> describing why efi_mm is safe.
>>>>
>>>> Details:
>>>>
>>>> * Registered with ptdump
>>>>       * ptep_get_lockless()
>>>> * efi_create_mapping -> create_pgd_mapping … -> init_pte:
>>>>       * __ptep_get()
>>>>       * __set_pte()
>>>> * efi_memattr_apply_permissions -> efi_set_mapping_permissions … ->
>>>> set_permissions
>>>>       * __ptep_get()
>>>>       * __set_pte()
>>>
>>> Sound good. We could add some VM_WARN_ON if we ever get the efi_mm via the
>>> "official" APIs.
>>
>> We could, but that would lead to the same linkage issue, which I'm trying to
>> avoid in the first place:
>>
>> VM_WARN_ON(IS_ENABLED(CONFIG_EFI) && mm == efi_mm);
>>
>> This creates new source code dependencies, which I would rather avoid if
>> possible.
> 
> Just a thought, you could have a is_efi_mm() function that abstracts all that.
> 
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index c74f47711f0b..152f5fa66a2a 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -692,6 +692,15 @@ extern struct efi {
>  
>  extern struct mm_struct efi_mm;
>  
> +static inline void is_efi_mm(struct mm_struct *mm)
> +{
> +#ifdef CONFIG_EFI
> +       return mm == &efi_mm;
> +#else
> +       return false;
> +#endif
> +}
> +
>  static inline int
>  efi_guidcmp (efi_guid_t left, efi_guid_t right)
>  {
> 
> 

That would definitely work, but in that case, I might as well just check for it
in mm_is_user() (and personally I would change the name to mm_is_efi()):


static inline bool mm_is_user(struct mm_struct *mm)
{
	return mm != &init_mm && !mm_is_efi(mm);
}

Any objections?