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Date: Tue, 13 Feb 2024 13:24:35 +0000
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Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings
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To: David Hildenbrand <david@redhat.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>
 <ZcoIVypNwOPIX30w@FVFF77S0Q05N>
 <c899f252-dbf3-4e7b-8342-b5a5180486cd@arm.com>
 <aa232591-e0c8-422d-a641-fa555914c5f0@arm.com>
 <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>
 <e89a3d11-fe73-4717-b0d6-55cc4fbe16cf@redhat.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <e89a3d11-fe73-4717-b0d6-55cc4fbe16cf@redhat.com>
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On 13/02/2024 13:22, David Hildenbrand wrote:
> 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.

Deal

>