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(p200300cbc73fce001be76d7f3cc3563d.dip0.t-ipconnect.de. [2003:cb:c73f:ce00:1be7:6d7f:3cc3:563d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438bd4b9984sm179722225e9.28.2025.01.28.12.24.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Jan 2025 12:24:34 -0800 (PST) Message-ID: Date: Tue, 28 Jan 2025 21:24:33 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [Question] Are "device exclusive non-swap entries" / "SVM atomics in Nouveau" still getting used in practice? To: Alistair Popple , "linux-mm@kvack.org" , John Hubbard , nouveau@lists.freedesktop.org, Jason Gunthorpe , DRI Development , Karol Herbst , Lyude Paul , Danilo Krummrich References: <346518a4-a090-4eaa-bc04-634388fd4ca3@redhat.com> <8c6f3838-f194-4a42-845d-10011192a234@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-MFC-PROC-ID: wfDFO1ZYIXaEwpXfJ4SPkCgfPGzggH7HOZhaxUq3aNI_1738095877 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: 8E8D28001C X-Stat-Signature: bxy6effforfmuzrrbr3rsa1qtw71g8xc X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1738095879-360392 X-HE-Meta: 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 DnPMSAAp 7017qp3XcKp1bkLddhlAFuhakeqtoKMv1YTg8zzew24kQC7eGIz+8W2NEFI4vFjnkIohQB5FBH/CA4iBMiLwMMbpnC0YZp0DxRARP+7FkChTgxxhdf5NwgG3rWd2Ksq2WEmmRp79F8qFUv/tz3zJQ34XAcPBupKv6MnMTs6aGQ1IIa7CVSK/qSiwwwSILnUd4RkqYDJPfThQAmOwmqAy9JH+V807g7YTU/0rLiLpn4Ji21llT8NsHkcLNT1i3+nPQOqDfdNRQb7IDN41j2CJvDbv1znnsEUQTvYLrn4IBbk8lkROuEJ/LkdiuUihwyVCgwON+SkZ+iftipOxm5VW/52W6GcIhv8cCXdKVwg2WdlYhsl0WVr4O1f1Bay5LMbuXqzKvUuV0N6s9vAG6ho192PVfs4PzQQpsF5aQKtT056vOtkqszE7QY3YNqnLbOQYdcLaZHXI+56Su4HlL3HCKjfkxWcg1/lAjC4+8 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000041, 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 28.01.25 21:14, Simona Vetter wrote: > On Tue, Jan 28, 2025 at 11:09:24AM +1100, Alistair Popple wrote: >> On Fri, Jan 24, 2025 at 06:54:02PM +0100, David Hildenbrand wrote: >>>>>> On integrated the gpu is tied into the coherency >>>>>> fabric, so there it's not needed. >>>>>> >>>>>> I think the more fundamental question with both this function here and >>>>>> with forced migration to device memory is that there's no guarantee it >>>>>> will work out. >>>>> >>>>> Yes, in particular with device-exclusive, it doesn't really work with THP >>>>> and is only limited to anonymous memory. I have patches to at least make it >>>>> work reliably with THP. >>>> >>>> I should have crawled through the implementation first before replying. >>>> Since it only looks at folio_mapcount() make_device_exclusive() should at >>>> least in theory work reliably on anon memory, and not be impacted by >>>> elevated refcounts due to migration/ksm/thp/whatever. >>> >>> Yes, there is -- in theory -- nothing blocking the conversion except the >>> folio lock. That's different than page migration. >> >> Indeed - this was the entire motivation for make_device_exclusive() - that we >> needed a way to reliably exclude CPU access that couldn't be blocked in the same >> way page migration can (otherwise we could have just migrated to a device page, >> even if that may have added unwanted overhead). > > The folio_trylock worries me a bit. I guess this is to avoid deadlocks > when locking multiple folios, but I think at least on the first one we > need an unconditional folio_lock to guarantee forward progress. At least on the hmm path I was able to trigger the EBUSY a couple of times due to concurrent swapout. But the hmm-tests selftest fails immediately instead of retrying. > Since > atomics can't cross 4k boundaries (or the hw is just really broken) this > should be enough to avoid being stuck in a livelock. I'm also not seeing > any other reason why a folio_lock shouldn't work here, but then my > understanding of mm/ stuff is really just scratching the surface. > > I did crawl through all the other code and it looks like everything else > is unconditional locks. So looks all good and I didn't spot anything else > that seemed problematic. > > Somewhat aside, I do wonder whether we really want to require callers to > hold the mmap lock, or whether with all the work towards lockless fastpath > that shouldn't instead just be an implementation detail. We might be able to use the VMA lock in the future, but that will require GUP support and a bunch more. Until then, the mm_lock in read mode is required. I was not able to convince myself that we'll really need the folio lock, but that's also a separate discussion. > At least for the > gpu hmm code I've seen I've tried to push hard towards a world were the > gpu side does not rely on mmap_read_lock being held at all, to future > proof this all. And currently we only have one caller of > make_device_exclusive_range() so would be simple to do. We could likely move the mmap_lock into that function, but avoiding it is more effort. In any case, I'll send something out probably tomorrow to fix page migration/swapout of pages with device-exclusive entries and a bunch of other things (THP, interaction with hugetlb, ...). -- Cheers, David / dhildenb