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[2003:cb:c719:1700:56dc:6a88:b509:d3f3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a1c8acadcsm50293217f8f.105.2025.01.07.08.55.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 07 Jan 2025 08:55:56 -0800 (PST) Message-ID: Date: Tue, 7 Jan 2025 17:55:54 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Non-lru page migration in a memdesc world To: Zi Yan Cc: linux-mm@kvack.org, lsf-pc@lists.linux-foundation.org References: <2612ac8a-d0a9-452b-a53d-75ffc6166224@redhat.com> <5C9489ED-0B26-40EA-B47B-C034E7DACB2F@nvidia.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: <5C9489ED-0B26-40EA-B47B-C034E7DACB2F@nvidia.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 1vIC8EvGlVRu0qC60VssenHh7TGCg60T4pTppQ6X2j0_1736268957 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 5B01914000B X-Rspam-User: X-Stat-Signature: 346kwxci4mxeokjcoxcgspuamg3kgc4a X-HE-Tag: 1736268960-266057 X-HE-Meta: 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 eT6Uqk/o uXadFw+NqiclffkZxQuhZMueOmc+qNOHXa+SC2eUiC1a6F5264vsurJpM0N+JLmNr+cQaIGWafv8dz0FrnHZXl3VtwLZYtCbATbZuf3DAbk0GiDuxWYgxJqGg7DQ0hfx/C+ieO1v5CjJiDy/TYdUlpSIEOCEMW4bPBHr9uEGZpY2JLlpl1TnmNrtmZ4QKINt0orDwXkRS/90qweZdq+brEERQpNWVKaJpFBXG6jz4VTYu4+D6xhAezk4kACMPA8xC2tem/Awp2dLNbAziHCyILAN9isrqPPIFSg9ew2ayOz1utEwFtZEeQlC0FukKjwoPVrZuBZruDYuekRKMAGIjA0bDWD1ccN7TwKp8/YnH4zt630GEwXWc3HcB/FVnZggqSw1cpmKSFFyfMGY= 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 07.01.25 17:48, Zi Yan wrote: > On 7 Jan 2025, at 11:11, David Hildenbrand wrote: > >> Hi, >> >> one item on my todo list is making PageOffline pages to stop using "struct page" members except page->type and 1/2 flags, to prepare them for the memdesc future, to avoid unnecessary atomics, and to resolve some (so-far) theoretical issues with temporary speculative references. >> >> For example, the page->_refcount will always be 0 (frozen) for PageOffline pages, and they will get allocated/freed similar to how we allocate/free frozen pages for slab already. Once we move the refcount into "struct folio", they will not have a refcount at all anymore. >> >> One complication is balloon compaction: we allow for migrating PageOffline pages allocated in some memory ballooning implementations such as virtio-balloon. >> >> For that, we use the "non-lru page migration" framework and in that process we make use of ... way to many members of "struct page"/"struct folio" and rely on the refcount not being 0. For example, we certainly don't want to allocate memdescs for PageOffline pages just so some of them can be migrated. > > Then first thing is to make all get_new_folio functions be aware of PageOffline > pages and be able to allocate a PageOffline page. IIUC, the current process > is: 1) allocate a page from buddy allocator, 2) offline the new page during > mops->migrate_page() and online the old page. The inflation and deflation > in step 2 looks redundant if migrate_pages() can get PageOffline pages to > begin with and put_page() can handle PageOffline page too. That might be one hacky way of handling offline pages, yes :) (the isolation step is tricky: for example, with page->lru gone we cannot even put these things into a list! Also, there is page isolation ...) I recall that the isolation step is required because we could have multiple parties trying to migrate the same page at the same time. So that must be handled as well. > >> >> While we converted non-lru page migration to work on folios (i.e., folio_movable_ops()) these things are not actually "folios" in the future, they can have different memdescs. >> >> So, how can we migrate non-lru things that are not folios while not relying on "struct folio" members, with minimal/no metadata overhead? > > Like I said above, if migrate_pages() is aware of PageOffline pages by allocating > and putting them like normal folios, that could work. > > Or you can do what hugetlb migration does, adding a separate migrate_offlinepages() > function to handle PageOffline pages. This probably can save you a lot of > LRU page checks like mapping and locks, but it adds a special function. So > tradeoffs. > >> >> I have some ideas, but no complete solution yet; input about the requirements of other non-lru page migration use cases besides PageOffline will be interesting. >> >> ... and maybe, we have other non-folio things we'd want to migrate, and want to be prepared to handle them as well? (hint: leaf page tables?) > > If we have dedicated allocator for non-folio things and make migrate_pages() > be aware of them, it should be doable. Note that I thought about similar things as you describe above, but part of the exercise will not be focusing on PageOffline pages, but having something more generic that can handle pages with actual page content, and that have to be properly isolated :) -- Cheers, David / dhildenb