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[91.12.107.64]) by smtp.gmail.com with ESMTPSA id o32-20020a05600c512000b004148a5e3188sm6115521wms.25.2024.03.26.10.58.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Mar 2024 10:58:56 -0700 (PDT) Message-ID: <6aaff470-c510-469b-8f4f-2e4c5cf07d56@redhat.com> Date: Tue, 26 Mar 2024 18:58:55 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 3/4] mm/memory: Use ptep_get_lockless_norecency() for orig_pte To: Ryan Roberts , Mark Rutland , Catalin Marinas , Will Deacon , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Andrew Morton , Muchun Song Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240215121756.2734131-1-ryan.roberts@arm.com> <20240215121756.2734131-4-ryan.roberts@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-Queue-Id: 15D9840003 X-Rspam-User: X-Stat-Signature: wy4rfjb8bf6kmijjxeyjomp7dbwwatqw X-Rspamd-Server: rspam01 X-HE-Tag: 1711475943-644908 X-HE-Meta: U2FsdGVkX1+QzboPwYnwoZvqLau404sgcyvvbTbdlVdc7Q5DY2g9O+mbOK+dxVCZ6basSek3AykXKTN8ugNhfe6nxJ9W2pKyDO41ZGCbyX8UxaDwmx76O64nqd9NbazF04GIZGeOneUYnztAD98KQA5Xj0UyZ0Wv5jqEQPWvFqQdm9QJBBYNoUvb9tFGBRkD9SPJMcxOqhxVrup2X4R9GE1glXNm1OyPBN40KYoxWKe19h/KRvwUi9JwP5UF/Po1lpvuKkSCvoR5grIir0YytJdab6JyjNhmrASDAEP4dI+LzQsjrPx3BswnykydZ6TuJa13vu8SaOTrqV/U2/JM+/0v59WBvQT3J4ttc7gM1vepwuu7KJS2uELRhApwBtNq3zIf/PjKnRAwlmqduTnwyPoOngLrv0aWLVvbpDy9u4WINtxJVtC1hQZrMl9yxXLhb8iJjM1NCZFbbzSD+I5zc6o7+IwABSDhlS+XwCQWs2Ave+wBg3C1bPyQmsmkRtKLIfPmO0imLopV1sIO2NzIXvQuedZbiRFouJYSxJ1K2RGzjfI6kZSQsJJd+b/G14hmOZcH+ghxJYBfHcAo6h5tL7wD/Qi0HuP8iTvnYzeksp0VffOVF5+UXX0JavGseQPZKWH/wgpQ3im0hOx/+mQ3NV8eL83FwVTjO3UpoVe8T+ucwTHMhyUXCKL8r0llNbVwhiYWkC/dPdOx5nR/ja62onTLCxv7Nqq3oUdoiypuFOZdkIZqnFXMgxbqdRv1vvnQbMqSqv1uptsMMOOW27+cHoePfPunuDTUzWvFja/WCjY8TZYHtm5m0vBShtpZ/ZIG952C15LPHAQZKxvdAcWnUZtg72Ka3zmEdFVIF2Y1iozWVZ3MYq9owGw2JEOxwcdK37U4NV2HiKGHqnwVnJffR8rt1SfHdOabwc7Z4NGFMpKhfnPxyVZ2yb7rr2Io9GJ7VjdzBSVDEfCLVuJvR6W SxM2x3k7 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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: >>>> >>>> vmf->orig_pte = ptep_get_lockless_norecency(vmf->pte) >>>> /* not dirty */ >>>> >>>> /* Now, thread 2 ends up setting the PTE dirty under PT lock. */ > > Ahh, this comment about thread 2 is not referring to the code immediately below > it. It all makes much more sense now. :) Sorry :) > >>>> >>>> spin_lock(vmf->ptl); >>>> entry = vmf->orig_pte; >>>> if (unlikely(!pte_same(ptep_get(vmf->pte), entry))) { >>>>      ... >>>> } >>>> ... >>>> entry = pte_mkyoung(entry); >>> >>> Do you mean pte_mkdirty() here? You're talking about dirty everywhere else. >> >> No, that is just thread 1 seeing "oh, nothing to do" and then goes ahead and >> unconditionally does that in handle_pte_fault(). >> >>> >>>> if (ptep_set_access_flags(vmf->vma, ...) >>>> ... >>>> pte_unmap_unlock(vmf->pte, vmf->ptl); >>>> >>>> >>>> Generic ptep_set_access_flags() will do another pte_same() check and realize >>>> "hey, there was a change!" let's update the PTE! >>>> >>>> set_pte_at(vma->vm_mm, address, ptep, entry); >>> >>> This is called from the generic ptep_set_access_flags() in your example, right? >>> >> >> Yes. >> >>>> >>>> would overwrite the dirty bit set by thread 2. >>> >>> I'm not really sure what you are getting at... Is your concern that there is a >>> race where the page could become dirty in the meantime and it now gets lost? I >>> think that's why arm64 overrides ptep_set_access_flags(); since the hw can >>> update access/dirty we have to deal with the races. >> >> My concern is that your patch can in subtle ways lead to use losing PTE dirty >> bits on architectures that don't have the HW-managed dirty bit. They do exist ;) > > But I think the example you give can already happen today? Thread 1 reads > orig_pte = ptep_get_lockless(). So that's already racy, if thread 2 is going to > set dirty just after the get, then thread 1 is going to set the PTE back to (a > modified version of) orig_pte. Isn't it already broken? No, because the pte_same() check under PTL would have detected it, and we would have backed out. And I think the problem comes to live when we convert pte_same()->pte_same_norecency(), because we fail to protect PTE access/dirty changes that happend under PTL from another thread. But could be I am missing something :) >> Arm64 should be fine in that regard. >> > > There is plenty of arm64 HW that doesn't do HW access/dirty update. But our > ptep_set_access_flags() can always deal with a racing update, even if that > update originates from SW. > > Why do I have the feeling you're about to explain (very patiently) exactly why > I'm wrong?... :) heh ... or you'll tell me (vary patiently) why I am wrong :) -- Cheers, David / dhildenb