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[2003:cb:c70a:1c00:b8d4:bc62:e7ed:ec0e]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c5c12247esm4209775f8f.53.2025.01.31.00.47.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 31 Jan 2025 00:47:41 -0800 (PST) Message-ID: Date: Fri, 31 Jan 2025 09:47:39 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] The future of ZONE_DEVICE pages To: Alistair Popple , linux-mm@kvack.org Cc: lsf-pc@lists.linux-foundation.org, willy@infradead.org, ziy@nvidia.com, jhubbard@nvidia.com, jgg@nvidia.com, balbirs@nvidia.com References: 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: KK3gXKVg-t9lt56lgs2sYrGtMRC1lW-fb0yslDzavXY_1738313262 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: 8A91D1C000C X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: z97wxoum3iwej3k67t6gi7tky4bi1i89 X-HE-Tag: 1738313265-256583 X-HE-Meta: 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 sXyAyGYP 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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 31.01.25 03:59, Alistair Popple wrote: > I have a few topics that I would like to discuss around ZONE_DEVICE pages > and their current and future usage in the kernel. Generally these pages are > used to represent various forms of device memory (PCIe BAR space, coherent > accelerator memory, persistent memory, unaddressable device memory). All > of these require special treatment by the core MM so many features must be > implemented specifically for ZONE_DEVICE pages. > > I would like to get feedback on several ideas I've had for a while: > > Large page migration for ZONE_DEVICE pages > ========================================== > > Currently large ZONE_DEVICE pages only exist for persistent memory use cases > (DAX, FS DAX). This involves a special reference counting scheme which I hope to > have fixed[1] by the time of the LSF/MM/BPF. Fixing this allows for other higher > order ZONE_DEVICE folios. > > Specifically I would like to introduce the possiblity of migrating large CPU > folios to unaddressable (DEVICE_PRIVATE) or coherent (DEVICE_COHERENT) memory. > The current interfaces (migrate_vma) don't allow that as they require all folios > to be split. > Hi, > Some of the issues are: > > 1. What should the interface look like? > > These are non-lru pages, so likely there is overlap with "non-lru page migration > in a memdesc world"[2] Yes, although these (what we called "non-lru migration" before ZONE_DEVICE popped up) are currently all order-0. Likely this will change at some point, but not sure if there is currently a real demand for it. Agreed that there is quite some overlap. E.g., no page->lru field, and the problem about splitting large allocations etc. For example, balloon-inflated pages are currently all order-0. If we'd want to support something larger but still allow for reliable balloon compaction under memory fragmentation, we'd want an option to split-before-migration (similar as you describe below). Alternatively, we can just split right at the start: if the balloon allocated a 2MiB compound page, it can just split it to 512 order-0 pages and allow for migration of the individual pieces. Both approaches have their pros and cons. Anyway: "non-lru migration" is not quite expressive. It's likely going to be: (1) LRU folio migration (2) non-LRU folio migration (->ZONE_DEVICE) (3) non-folio migration (balloon,zsmalloc, ...) (1) and (2) have things in common (e.g., rmap, folio handling) and (2) and (3) have things in common (e.g., no ->lru field). Would there be something ZONE_DEVICE based that we want to migrate and that will not be a folio (iow, not mapped into user page tables etc)? > > 2. How do we allow merging/splitting of pages during migration? > > This is neccessary because when migrating back from device memory there may not > be enough large CPU pages available. > > 3. Any other issues? > > [1] - https://lore.kernel.org/linux-mm/cover.11189864684e31260d1408779fac9db80122047b.1736488799.git-series.apopple@nvidia.com/ > [2] - https://lore.kernel.org/linux-mm/2612ac8a-d0a9-452b-a53d-75ffc6166224@redhat.com/ > > File-backed DEVICE_PRIVATE/COHERENT pages > ========================================= > > Currently DEVICE_PRVIATE and DEVICE_COHERENT pages are only supported for > private anonymous memory. This prevents devices from having local access to > shared or file-backed mappings instead relying on remote DMA access which limits > performance. > > I have been prototyping allowing ZONE_DEVICE pages in the page cache with > a callback when the CPU requires access. Hmm, things like read/write/writeback get more tricky. How would you writeback content from a ZONE_DEVICE folio? Likely that's not possible. So I'm not sure if we want to go down that path; it will be great to learn about your approach and your findings. [...] There is a lot of interesting stuff in there; I assume too much for a single session :) -- Cheers, David / dhildenb