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Date: Tue, 13 Feb 2024 14:13:26 +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>,
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 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
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 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
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 x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org,
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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <1d302d7a-50ab-4ab4-b049-75ed4a71a87d@arm.com>
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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