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Date: Tue, 13 Feb 2024 14:22:46 +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>,
 James Morse <james.morse@arm.com>, Andrey Ryabinin <ryabinin.a.a@gmail.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>,
 Kefeng Wang <wangkefeng.wang@huawei.com>, John Hubbard
 <jhubbard@nvidia.com>, Zi Yan <ziy@nvidia.com>,
 Barry Song <21cnbao@gmail.com>, Alistair Popple <apopple@nvidia.com>,
 Yang Shi <shy828301@gmail.com>, Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 "H. Peter Anvin" <hpa@zytor.com>, linux-arm-kernel@lists.infradead.org,
 x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20240202080756.1453939-1-ryan.roberts@arm.com>
 <20240202080756.1453939-20-ryan.roberts@arm.com>
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 <c899f252-dbf3-4e7b-8342-b5a5180486cd@arm.com>
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 <64395ae4-3a7d-45dd-8f1d-ea6b232829c5@arm.com>
 <d6ce951f-f83b-4a5a-a814-311f2d8b01e4@arm.com>
 <41499621-482f-455b-9f68-b43ea8052557@redhat.com>
 <1d302d7a-50ab-4ab4-b049-75ed4a71a87d@arm.com>
 <99e2a92c-f2a2-4e1e-8ce2-08caae2cb7e4@redhat.com>
 <dce5f80d-942f-439c-a549-5290666464ca@arm.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <dce5f80d-942f-439c-a549-5290666464ca@arm.com>
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On 13.02.24 14:20, Ryan Roberts 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?
> 

Nope :) Maybe slap in an "unlikely()", because efi_mm *is* unlikely to 
show up.

-- 
Cheers,

David / dhildenb