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[2003:cb:c715:6600:a077:c7da:3362:6896]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3821ae161cdsm1208726f8f.79.2024.11.14.03.18.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 Nov 2024 03:18:16 -0800 (PST) Message-ID: Date: Thu, 14 Nov 2024 12:18:15 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird 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 To: "linux-mm@kvack.org" Subject: PageOffline: refcount, flags and memdesc Cc: Matthew Wilcox X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: k_VHRqPDlJ6itM3RHvtzonZ34EOfqaYOylfM3QfcHdk_1731583098 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: rspam09 X-Rspamd-Queue-Id: F209818000D X-Stat-Signature: ttt3synur6k6g4m98edbur7794kx95xc X-Rspam-User: X-HE-Tag: 1731583095-537383 X-HE-Meta: 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 fLiT5z3e +QGG5cOK29oBXDgg9YDnsRBW39gVYmJ7GTOOoKyIJWaYeS8ALEIHLqf453WBLr+fpIJ1z2E0/z5+lSoSdypB04MHDSwiBEAGVRr1YL2X0gkb/0z0pmGx0oIMZhuLztLWRUUxues+54PfpZjEgScQdXQdZdbkYZheJSsWoUv0wNMtNcZzw7BK+Rn7yBy/H4SF/5k86cBjaJbd4M6MlXvrNjNt3Z9BBaiMsZMXEVK3GW7UG8dLBm8EosDSUM01QIDapWTCJ+ZPM7SgvKofQqXAYO33XDHsWe6zyWC8mPKoUSk4k9dzwhBxLVHZUVhRaSLT0jENOaGI3RORyR1XFpx47J90NL2ORRPTM8rBdwidQSgSi3iqDgCXYlE0WBBqKdtCupc57pWGewkB8iC5AOD4nuGyUvUGFMrC9mTbqAK4upUnUyKJlboCxbf67eQ== 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: Hi, I'm currently staring again at PageOffline and wonder how we could prepare it for the memdesc future, and if we can remove refcount handling. Currently, we set PageOffline in the following cases (one nast exception below): (a) Memory blocks gets onlined, whereby we initialize all "struct pages to PageOffline + refcount of 1: memmap_init_range(). These pages are expected to get onlined via generic_online_page() later. Drivers might decide to leave some offline, because they are not backed by actual memory in the hypervisor. Some drivers still use free_page() instead of generic_online_page(). (b) We allocated pages (alloc_page(), alloc_contig_pages() ...) to logically offline them, whereby the refcount is set to 1 by the buddy and to PageOffline is set manually be the driver afterwards. We clear PageOffline in the following cases (one nasty exception below): (a) We want to return a page to the buddy (free_page/ free_contig_page_range). PageOffline is cleared by the driver and freeing the page will decrement the refcount to 0. (b) We want to expose it to the buddy the first time (generic_online_page). We will force the refcount to 0. There are still subtle differences between onlining a page the first time to the buddy, such as debug_pagealloc_map_pages() in __free_pages_core(). I'm hoping we can get rid of them long-term, or just abstract it internally. I'd like to stop using the refcount for PageOffline pages, and keep the refcount always at 0. But the refcount, it is currently used to detect whether we are allowed to offline memory blocks that contain PageOffline pages, because only selected drivers support re-onlining. Well, and it is used when returning the pages to the buddy where free_page()/free_contig_range().... expect a refcount of 1. Further, virtio-mem currently uses the PageDirty() bit to remember if a PageOffline page was already exposed to the buddy before, or if we must use generic_online_page(). For now we would need the following information, that could be stored in 2 flags, leaving the refcount at 0: (1) Was it obtained from the buddy or never exposed it to the buddy PageOffline() && PageOfflineNeverOnlined() (2) The driver does support actual memory offlining+reonlining, they can be skipped when offlining. PageOffline() && PageOfflineSkippable But when allocating/freeing pages we would still mess with the refcount, which is bad. We could have a dedicated interface for freeing them, where we abstract the generic_online_page() bits, and leave the refcount at 0: free_offline_page() free_offline_page_range() And alloc_offline_page() alloc_offline_page_range() alloc_offline_pages I'm not super happy about the "alloc/free" terminology, but nothing better came to mind. There is one complication to sort out: balloon_compaction.h supports moving PageOffline pages, and seems to use the page lock, page refcount, page lru, page private... which is all rather nasty. I wonder if these should get their own page type, like PageMovableOffline, and we'd mostly leave them alone for now. This would mean that virtio-balloon, vmware-balloon and ppc CMM would keep doing the old refcount-based thing but with a new page type. I assume this all goes into the direction of getting pages from the buddy and returning them without refcounts ... thoughts? -- Cheers, David / dhildenb