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Tue, 09 Jul 2024 21:05:38 -0700 (PDT) Received: from [172.20.2.228] ([4.28.11.157]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-70b4396740asm2662984b3a.105.2024.07.09.21.05.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Jul 2024 21:05:36 -0700 (PDT) Message-ID: <9b400450-46bc-41c7-9e89-825993851101@redhat.com> Date: Wed, 10 Jul 2024 06:05:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings From: David Hildenbrand To: "Jason A. Donenfeld" , linux-kernel@vger.kernel.org, patches@lists.linux.dev, tglx@linutronix.de Cc: linux-crypto@vger.kernel.org, linux-api@vger.kernel.org, x86@kernel.org, Linus Torvalds , Greg Kroah-Hartman , Adhemerval Zanella Netto , Carlos O'Donell , Florian Weimer , Arnd Bergmann , Jann Horn , Christian Brauner , David Hildenbrand , linux-mm@kvack.org References: <20240709130513.98102-1-Jason@zx2c4.com> <20240709130513.98102-2-Jason@zx2c4.com> <378f23cb-362e-413a-b221-09a5352e79f2@redhat.com> 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: <378f23cb-362e-413a-b221-09a5352e79f2@redhat.com> 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: 7bit X-Rspamd-Queue-Id: A5BC11C0003 X-Stat-Signature: jjbunuojn8ija1am3981nkwyc68eczmg X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1720584342-739887 X-HE-Meta: U2FsdGVkX1/UKzll2rBNz5Csl5ivBi8cu++jSWa5NRaYgIr+ubyKzeKrtvdqZUx12UMpcSwWvnJ1M1yMzJn1+syFwbYkjFXkPe+GdzWAscB5NHOuH2HRFHe1xXD13mbeWQl3gtSBf06QVZB2Z6p69UyaElMBTCLZsJxMlGGAR8zkpT+D72SfuaobUi5mUwzq5LydiRsFH+/D9FBJURgcHF4pBC6alAcSTn6dLII4YeENObsQT7ISViJ91bduFvZoVMPJ7+6uFdsVJ80aqv2UVW8SSIHUycjDz85Zu+rh4Cm3JApWnYZ/xCbMoNh6FNDQG+VBoOnbNwM2S7vVWzxBfZ9OVrPuYs8bZKtavRNcorq4CHlc+HVcTt8+Tj5ER2DJFKAXVWF+K1hfS0vcwVMxnWJ//tQd7aYwjekfzg13rbRA/VgXIuVe2yzPjOrHUl+m5HqjSmxhWy5M8CuDzdu1/zsAKmKUfT4DukBhqCFycI6t5L+PWCqd8z+VgqXD9ur/mC40dYDLnb0XkcXT+CC+SkpRBiu7vBMMp1NpH88O71DYm/D2TOXaR2M4npzB0nBBdDKIjHZ6uWfTpdogIlvGkb+MLcjCZ4+tRnJKUUlbhbPPW4Wa5VyOQV/Gwh559M+poTWq4+Iud7CGGnL4+uKyeWZ1yp55Gz4KtYKc/DtSkLxBn03g5eR08pVx1k02FfCe6H1qVliyOMqmRPFRF/s/O54pyvE7P8yiloKd3OwBYMhL4i56kydsH3SRQzQRpQnkb6Cc9f4kvhHf1G52sScjnwDuu1tItTSs2H0sSveLhiKUOtmwxNwd80yUfza5lnNCp34EaxPNQ/4lGkrZrcMhZxg45mEVChFFjcChXVWrZ/0D+/SdOCrwKRX9hEDofXhRc8zQ+0OBvOV9LhAcA8YxQPoLClDTZXace0vx4GgIyeJPpMZEEHYOxTiupUGI6xA+rKzRzOLhKeVgssausTw RXi4hyKy 0x/xMRuQQU4ZqP21tHAMmPE8TJLs7s9sZTAxj2iHRVDJyv5PTJpb2e1KyjBl7fBlSrViet66NcDI/yReTpMjFogaFQbp9U0n6PlChJCZH6xEp1AB8ikqYBprTu2StmOW9c6IUmabNHiuYWBo4/CuDkoZ5MarBwSLs+gW1dM5Bpo1FpdnPAtjp4j0Kif/w7liV8TNxfE/Gx8T6Nvglb91jotOhRKew6j4E2daOjoTW2X1KmnshDTE7g3jtBDAF4NK0sHTPrqsJEi8pSHjLRYu7NOziDetnHVILGkR1qP4m8VWCtza8/l+drlBggnSFOAIg64uNZWKMHksiNKASmE1/Ez0osT2aB0npUjJDzlJnWF1Jz9MckA7tTPXdE1jvIxFF3AuIzKnUK0aH1c1ozLStMw0i5nhbtz9Vth5+KQJ/qDrGylWD1+abOZ+wTBaWd/fHfIa8tVkTIwYRvfQ= 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 10.07.24 05:27, David Hildenbrand wrote: > On 09.07.24 15:05, Jason A. Donenfeld wrote: >> The vDSO getrandom() implementation works with a buffer allocated with a >> new system call that has certain requirements: >> >> - It shouldn't be written to core dumps. >> * Easy: VM_DONTDUMP. >> - It should be zeroed on fork. >> * Easy: VM_WIPEONFORK. >> >> - It shouldn't be written to swap. >> * Uh-oh: mlock is rlimited. >> * Uh-oh: mlock isn't inherited by forks. >> >> It turns out that the vDSO getrandom() function has three really nice >> characteristics that we can exploit to solve this problem: >> >> 1) Due to being wiped during fork(), the vDSO code is already robust to >> having the contents of the pages it reads zeroed out midway through >> the function's execution. >> >> 2) In the absolute worst case of whatever contingency we're coding for, >> we have the option to fallback to the getrandom() syscall, and >> everything is fine. >> >> 3) The buffers the function uses are only ever useful for a maximum of >> 60 seconds -- a sort of cache, rather than a long term allocation. >> >> These characteristics mean that we can introduce VM_DROPPABLE, which >> has the following semantics: >> >> a) It never is written out to swap. >> b) Under memory pressure, mm can just drop the pages (so that they're >> zero when read back again). >> c) It is inherited by fork. >> d) It doesn't count against the mlock budget, since nothing is locked. >> >> This is fairly simple to implement, with the one snag that we have to >> use 64-bit VM_* flags, but this shouldn't be a problem, since the only >> consumers will probably be 64-bit anyway. >> >> This way, allocations used by vDSO getrandom() can use: >> >> VM_DROPPABLE | VM_DONTDUMP | VM_WIPEONFORK | VM_NORESERVE >> >> And there will be no problem with using memory when not in use, not >> wiping on fork(), coredumps, or writing out to swap. >> >> In order to let vDSO getrandom() use this, expose these via mmap(2) as >> MAP_DROPPABLE. >> >> Finally, the provided self test ensures that this is working as desired. > > Acked-by: David Hildenbrand > > > I'll try to think of some corner cases we might be missing. BTW, do we have to handle the folio_set_swapbacked() in sort_folio() as well? /* dirty lazyfree */ if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio)) { success = lru_gen_del_folio(lruvec, folio, true); VM_WARN_ON_ONCE_FOLIO(!success, folio); folio_set_swapbacked(folio); lruvec_add_folio_tail(lruvec, folio); return true; } Maybe more difficult because we don't have a VMA here ... hmm IIUC, we have to make sure that no folio_set_swapbacked() would ever get performed on these folios, correct? -- Cheers, David / dhildenb