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

[Ryan Roberts]

On 13/02/2024 13:22, David Hildenbrand wrote:
> 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.

Deal

>