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[79.242.51.46]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c2a1bad92sm3327090f8f.61.2025.01.24.09.54.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 24 Jan 2025 09:54:03 -0800 (PST) Message-ID: Date: Fri, 24 Jan 2025 18:54:02 +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: "linux-mm@kvack.org" , John Hubbard , nouveau@lists.freedesktop.org, Jason Gunthorpe , Alistair Popple , 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: gdgsw-zUAo-MuYy-R5O1iChKrTSJixojyDMEEmeF45s_1737741245 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: 6iheykc6sfopaynufqugp8ahtz3nfwht X-Rspam-User: X-Rspamd-Queue-Id: 07CE020007 X-Rspamd-Server: rspam03 X-HE-Tag: 1737741247-693927 X-HE-Meta: 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 /e0JBmWG 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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 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. [...] > >> Then, we seem to give up too easily if we cannot lock the folio when wanting >> to convert to device-exclusive, which also looks rather odd. But well, maybe >> it just works good enough in the common case, or there is some other retry >> logic that makes it fly. > > I've crawled through the path to migrate pages from device memory back to > system memory a few months ago, and found some livelock issues in there. > Wouldn't be surprised if m_d_e has some of the same, but I didn't dig > through it (least because intel can't use it because not-so-great hw > design). Yes, that might be possible. Maybe something keeps spinning while the folio is locked instead of properly waiting for the lock. ... or someone is trying to convert a tail page of a THP, in which case we will also keep failing the conversion right now. >> There are other concerns I have (what if the page is pinned and access >> outside of the user space page tables?). Maybe there was not need to handle >> these cases so far. > > I think that's also ok, but might be good to document this clearly that > concurrent direct i/o or rdma registered buffer or whatever will mess with > this. The promise is only between the gpu and the cpu, not anything else, > in current apis. At least to my knowledge. Well, the issue is that e.g., iouring fixed buffers can be accessed from the CPU using the direct map from the CPU, not necessarily using DMA from a device. In any case, I'm planning on adding some code-level documentation for that and look into extending the high-level doc while at it. > >> So best I can do is make anonymous memory more reliable with >> device-exclusive and fixup some of the problematic parts that I see (e.g., >> broken page reclaim, page migration, ...). >> >> But before starting to cleanup+improve the existing handling of anonymous >> memory, I was wondering if this whole thing is getting used at all. > > Yeah if this can be made reliable (under the limitation of only anon > memory and only excluding userspace access) then I expect we'll need this > for a very long time. I just had no idea whether even that is possible. > > What isn't good is if it's only mostly reliable, like the current > pgmap->ops->migrate_to_ram() path in do_swap_page() still is. I'll cc you on patches once I figure out some details on how to fix some page table walkers that really don't expect these non-swap entries. Fortunately, the hmm test device is in place to trigger some shaky scenarios. -- Cheers, David / dhildenb