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

David Hildenbrand
2024-02-13T13:22:57Z

On 13.02.24 14:20, Ryan Roberts wrote:
> 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?
>

Nope :) Maybe slap in an "unlikely()", because efi_mm *is* unlikely to
show up.

--
Cheers,

David / dhildenb