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

[Ryan Roberts <ryan.roberts@arm.com>]

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()

Thanks,
Ryan

> 
> Thanks,
> Ryan
>