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

[Ryan Roberts]

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.