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Date: Tue, 13 Feb 2024 13:19:08 +0100
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Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings
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To: Ryan Roberts <ryan.roberts@arm.com>, Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>,
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From: David Hildenbrand <david@redhat.com>
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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.

-- 
Cheers,

David / dhildenb