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

David Hildenbrand
2024-02-13T13:13:33Z

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)
{

--
Cheers,

David / dhildenb