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

[David Hildenbrand <david@redhat.com>]

On 13.02.24 15:02, Ryan Roberts wrote:
> On 13/02/2024 13:45, David Hildenbrand wrote:
>> On 13.02.24 14:33, Ard Biesheuvel wrote:
>>> On Tue, 13 Feb 2024 at 14:21, Ryan Roberts <ryan.roberts@arm.com> 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?
>>>>
>>>
>>> Any reason not to use IS_ENABLED(CONFIG_EFI) in the above? The extern
>>> declaration is visible to the compiler, and any references should
>>> disappear before the linker could notice that efi_mm does not exist.
>>>
>>
>> Sure, as long as the linker is happy why not. I'll let Ryan mess with that :)
> 
> I'm not sure if you are suggesting dropping the mm_is_efi() helper and just use
> IS_ENABLED(CONFIG_EFI) in mm_is_user() to guard efi_mm, or if you are suggesting
> using IS_ENABLED(CONFIG_EFI) in mm_is_efi() instead of the ifdefery?
> 
> The former was what I did initially; It works great, but I didn't like that I
> was introducing a new code dependecy between efi and arm64 (nothing else outside
> of efi references efi_mm).
> 
> So then concluded that it is safe to not worry about efi_mm (thanks for your
> confirmation). But then David wanted a VM_WARN check, which reintroduces the
> code dependency. So he suggested the mm_is_efi() helper to hide that... This is
> all starting to feel circular...

I think Ard meant that inside mm_is_efi(), we could avoid the #ifdef and 
simply use IS_ENABLED().

-- 
Cheers,

David / dhildenb