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

Ryan Roberts
2024-02-13T14:02:07Z

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...

Since I've jsut updated the code to do it David's way, I propose leaving it as
is since nobody has strong feelings.

>
>> In any case, feel free to add
>>
>> Acked-by: Ard Biesheuvel <ardb@kernel.org>

Great thanks!

>
> Thanks for the review.
>