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Peter Anvin" , 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> <64395ae4-3a7d-45dd-8f1d-ea6b232829c5@arm.com> <41499621-482f-455b-9f68-b43ea8052557@redhat.com> <1d302d7a-50ab-4ab4-b049-75ed4a71a87d@arm.com> <99e2a92c-f2a2-4e1e-8ce2-08caae2cb7e4@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: A586A10001D X-Stat-Signature: w771nwmoassgyc7h3ybktoena13kaant X-HE-Tag: 1707831938-524476 X-HE-Meta: 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 mrjgQkEj 0GYmGdAC6+m1TSFPuqKGKtP/Xw0DsycIHiiiaVvWtWgqKR8AuwI9p3zhU7zi7L+0o91zbuAsM0tLBUOoCreG8AFFxWv7lf4MgMKEY688pOXnfN3EG4LLgtbhMkp7r44KUjeYRRsrpW5usV1dbyioVOYEr8Ho7Qk02/9Anc4xBF4Hr3EduaRgj/niVzBN527PMTZ5d5PJTcC5eRUznLiVi/9SstW+nD6+UbCCLV33l9J858wHnZzmko7IhtPS73le39bZyvuQEKf/aP8Nkfcki1lgJkF78OFOMRZlN X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 13.02.24 14:33, Ard Biesheuvel wrote: > On Tue, 13 Feb 2024 at 14:21, 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? >> > > Any reason not to use IS_ENABLED(CONFIG_EFI) in the above? The extern > declaration is visible to the compiler, and any references should > disappear before the linker could notice that efi_mm does not exist. > Sure, as long as the linker is happy why not. I'll let Ryan mess with that :) > In any case, feel free to add > > Acked-by: Ard Biesheuvel Thanks for the review. -- Cheers, David / dhildenb