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[2003:cb:c715:6600:a077:c7da:3362:6896]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3821adbbc44sm1836156f8f.45.2024.11.14.07.55.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 Nov 2024 07:55:34 -0800 (PST) Message-ID: <1239f9a1-81b7-4752-bfa4-d3ec9bd9118f@redhat.com> Date: Thu, 14 Nov 2024 16:55:33 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: PageOffline: refcount, flags and memdesc To: Matthew Wilcox Cc: "linux-mm@kvack.org" 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: mFH7aquOMzT_GRW_kSsceZMqRVyedR9TZfMN8MLTel8_1731599736 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: rspam04 X-Rspamd-Queue-Id: BDE38A0007 X-Stat-Signature: o8brrd3uwwz5cbt1x4uhdjodzb1pkjuu X-Rspam-User: X-HE-Tag: 1731599706-674635 X-HE-Meta: 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 D6ctxAk8 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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 14.11.24 16:23, Matthew Wilcox wrote: > On Thu, Nov 14, 2024 at 12:18:15PM +0100, David Hildenbrand wrote: >> 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. > Hi! > Thanks for bringing it up. As a memdesc, I currently have PageOffline > as being type 0 (Misc), subtype 5 (Offline). That's bits 0-10 and then > bit 11 is for "may be mapped to userspace". Bits 12-17 are the order. > With the top bits being used for section/node/zone, that could be 25 + > 12 + 3 = 40 bits, so we'd have 7 bits remaining for use as flags. "may be mapped to userspace" will always be 0. 1 / 2 flags initially would do. > >> I'd like to stop using the refcount for PageOffline pages, and keep the >> refcount always at 0. > > I think this makes sense. > >> 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. > > If I resurrect > https://lore.kernel.org/linux-mm/20220809171854.3725722-1-willy@infradead.org/ > would the frozen terminology work for you here? Ah, I remember that. I was more concerned about alloc/free terminology, because "free_offline_page" could simply be "online_page" :) But the "allocation" part is trickier. Maybe it's simply alloc/free of frozen pages for the time being. But yes, the "allocate/free pages without involving refcounts" will be a crucial thing to get the PageOffline conversion flying. Instead of alloc_frozen_pages(), I was wondering if we should have something like GFP_FROZEN. For example, for two PG_offline users I'd currently also need alloc_contig_frozen_range() and alloc_contig_frozen_pages(). Using alloc_contig_range(GFP_FROZEN) alloc_contig_pages(GFP_PROZEN) would make that easier. Did you consider that already? > >> 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. > > It's fairly clear to me now that we have a sane story for moving > file/anon folios. The current way we handle movable pages looks mostly > insane because it's hammered into that framework, I think we need > something entirely different to handle movable non-folio pages, but I > don't know what that story is yet. Okay, so the first step would be to leave that part alone and convert the other (sane :) ) users of PageOffline to not refcount. -- Cheers, David / dhildenb