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[2003:cb:c72d:2500:94b5:4b7d:ad4a:fd0b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-397f2837e61sm12310195f8f.97.2025.03.18.03.12.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Mar 2025 03:12:27 -0700 (PDT) Message-ID: <6bb178de-67d1-41fd-a033-72fb52ad0905@redhat.com> Date: Tue, 18 Mar 2025 11:12:24 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: A plan for supporting PageMovable in 2025 To: Matthew Wilcox , 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: PVY2Db9e_ErStI1ZsdDzjXvEQxQvdV7AK8Ggx0NflsI_1742292749 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: qghsheg8t8p3zagmi57hqzp9torn1rm4 X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 7475B1C0007 X-Rspam-User: X-HE-Tag: 1742292751-778167 X-HE-Meta: U2FsdGVkX1+J8q3Xvh0eTwz4ddHtfhUsac/yQjygccPPQYofevwCeO1iuyKHkyL0W5DqtF47mlKQpdTAjaIoqFiehe/nawx+amG56IxtOa/FAZ/2LGMoYb4FKvLeV9UWc7aTiXr8nsoGqvVbIINwXZ+KjD+1FUmML/9bryzmnfEhjxk5B/vDX9zZutD9O1+vOFT//dhk0GH1mdFbSCD1wx3UnBKei92gopKfow7GZcjBFEmeV/cF6qya+9k3jope0k5buz7v0YiUIpV5uw2xXFvM+iHzsGhPxOjzStxJ3DWzsumZ7MAEDtM121ughl0MucXfi9n43yeOP2kYwKp0JEpkIyjU3D0L5eyw54LJukoA+W4jYCUF+a6IghvaKVOl//Ybi53sKl/kdjrRSTppdqKoApvPhL7y5w7/7RHu80vY/phKHr0ZW4pM7RJC+y9FA3ihAmBI6P7NqL3RRs0perVJ8NwXcjh+r33KaDg1Vl1uuUhj+xtwMFf3wy0H0VJZwbciM4CKip9Awn2QNdIUMBlvynQpuItCdERZ8Qu/0ZagiohR2OB0sP3/17aRvcpEc0H8ZtkLLgRDR/ZgW5hpuqU40K5fgRryUtpAyIi+TD6VjU3kfq9/1qrAKLf/w1RFGA/90LO+nN019DjCTCRblkCb3wavN9I/nzd8tEvnLq8ElcKpj6wC1InFFcEb5YiI7MYGNa3fr97SwMGS5XYT297z+TGpFndDEfxx1FixPMjMcRUPKeJ0AzVrYNoCHeqjWesmooUtV5UUCtYVTErWp+379u4NSVtkgp28VGfpuiB2u92+I/exl4k1wy2YfCuvPPZhXUNG9XZgM/4x4IBUKpWIRseRheo2h5ML59oB6TM+inSeLiEFdxLU/BFGHzg+N5Db9oceIIRDdM98PEBGqJX5CHISoz9E+uV6mbNaa6ZND3cah4Oh8mf6Cz7gWOoh0AWlWLx5X2pPO5go3wT Rvv8yE8o IA86RAfvQNxUE2oBWO0ufoI1GRqwMQno+NO13vzg4BnrRZpgWnPs5rk8SvdPL08CoKKtS3B1desEHpt4wlkqzLun57MqC6Q3SOThCtZxBCPKTY443qkpst+p4vtPHLzjsKlQMC+c0C3fD2wke6UCt+C8qzcOQGrHAOlmCjnlxDvsgJ4Nh8Nsg3mRP7hci3AbQbpr3mBXD3YSJeO049zJc424ciTnBoxBSgoMTK9RUjF8DTtlJ/SL0gl7MfhD1MB5U8JJ9ZtprOzSFjpPd/ovao5unQqTzEscdD9lCX6smJs7v4n5X2Rq5dwOhXaki5i4IhuqQcc5wZs3oLk6qCuSukZrbc6AIpLGuICY6OuqvxbhYtkRILShXKpd+xbf91iBCJsx80CpMGWqVzCdPDDGL2/pv/w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000023, 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 17.03.25 18:00, Matthew Wilcox wrote: > With the upcoming shrink of struct page to 4 words, we need a plan for > handling PageMovable. Ideally this does not involve memory allocation, > and is a relatively simple change from what we have now. To shrink > struct page beyond 4 words, we'll need a better plan, but I think this > will do for the next few months. Right, I've been focusing on grasping what we need in the long run with frozen pages that don't even want any memdesc (PageOffline). > > The current proposed layout for struct page is: > > struct page { > unsigned long flags; > union { > struct list_head buddy_list; > struct list_head pcp_list; > struct { > unsigned long memdesc; > union { > unsigned long private; > atomic_t _mapcount; > }; > }; > }; > int _refcount; > }; > > My proposal for movable non-folio pages is: > > * memdesc is used to point to struct movable_operations (these will > need to be aligned to 16 bytes, but I think that's fine) Note that we don't want to allocate a memdesc for PageOffline pages in the long run. For balloon compaction it might be fine as a first step. How'd we handle PAGE_MAPPING_MOVABLE? See below on my idea to avoid what you describe here. > * private is used to point to the next page in the list > * These pages are refcounted > * We retain a "lock" bit in page->flags Note that there is also PG_isolated, which I am hoping we can get rid of. My current bigger idea is something like this: 1) memdesc type (currently folio type) identifies "struct movable_operations". We could think of a registration model for migration handlers. Pg_offline -> call into balloon compaction Calling the ->isolate callback will fail if the callback is not responsible for migrating the page, or if somebody else already isolated it. Ideally, we'd have two bits (per memdesc) to essentially indicate "this is movable" and "this is isolated". Not 100% sure if the latter is required. If already isolated, simply calling the ->isolate callback will fail. I think most of the existing PG_isolated users are irrelevant, but it's all complicated. So a single per-memdesc bit + memdesc type might be sufficient to lookup the 2) No dependency on the refcount: ->isolate / ->putback effectively move the ownership ("reference") from the real owner to migration code (so they can be frozen). We just have to make sure that, while a page is isolated, that it cannot be freed by the real owner. (which is already the case IIRC) 3) No lists: we simply use an array of PFNs in migration code? 4) Lock bit: not 100% sure yet, but likely not required if ->isolate / ->migrate / ->putback just handle this locking internally. Lists are a problem for ballooning drivers with PageOffline pages. I had the exact same thought as you regarding "private is used to point to the next page in the list", but discarded it because it's inefficient for ballooning purposes and not future proof. So instead, my plan is to using an xarray in the ballooning drivers to store the PFNs of inflated pages. The only nasty thing is that "insert page in the balloon" can fail if OOM (inserting into the xarray). In general, that's just fine, except in some XEN / Hyper-V code where PageOffline pages are not allocated from the buddy where we could put them back, but they "come to life" with memory that gets added. -- Cheers, David / dhildenb