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[2003:cb:c708:8a00:362b:7e34:a3bc:9ddf]) by smtp.gmail.com with ESMTPSA id l20-20020a05600c1d1400b004146a304863sm2703001wms.34.2024.03.27.10.05.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 Mar 2024 10:05:53 -0700 (PDT) Message-ID: <44fc26ae-de7d-4bed-af7b-bcd2c593c676@redhat.com> Date: Wed, 27 Mar 2024 18:05:52 +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> <6aaff470-c510-469b-8f4f-2e4c5cf07d56@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-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 45779C0020 X-Stat-Signature: y9m1o5k6ar75kq7oabsu63d31k1dtc8e X-Rspam-User: X-HE-Tag: 1711559159-349314 X-HE-Meta: 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 Zftx75rg 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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 27.03.24 10:51, Ryan Roberts wrote: > On 26/03/2024 17:58, David Hildenbrand wrote: >>>>>> >>>>>> 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. > > Ahh yep. Got it. I absolutely knew that you would be correct, but I still walked > right into it! > > I think one could argue that the generic ptep_set_access_flags() is not > implementing its own spec: > > " > ... Only sets the access flags (dirty, accessed), as well as write permission. > Furthermore, we know it always gets set to a "more permissive" setting ... > " > > Surely it should be folding the access and dirty bits from *ptep into entry if > they are set? Likely yes. Unless it's also used to clear access/dirty (don't think so, and would not be documented). But the simplification made sense for now, because you previously checked that pte_same(), and nobody can modify it concurrently. > > Regardless, I think this example proves that its fragile and subtle. I'm not > really sure how to fix it more generally/robustly. Any thoughts? If not perhaps > we are better off keeping ptep_get_lockless() around and only using > ptep_get_lockless_norecency() for the really obviously correct cases? Maybe one of the "sources of problems" is that we have a ptep_get_lockless_norecency() call followed by a pte_same() check, like done here. Not the source of all problems I believe, though ... -- Cheers, David / dhildenb