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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> 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-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 2053EC000F X-Stat-Signature: sh7y8oqwjr7gckkkhz3sbwwifa5gq55a X-Rspam-User: X-HE-Tag: 1707826753-462817 X-HE-Meta: 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 5SY4AAFm 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 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 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