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(p200300d82f1f5e00c941d6fb3e300b42.dip0.t-ipconnect.de. [2003:d8:2f1f:5e00:c941:d6fb:3e30:b42]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-458705684a8sm11879845e9.26.2025.07.24.01.59.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 24 Jul 2025 01:59:19 -0700 (PDT) Message-ID: <86516155-f2d9-4e8d-9d27-bdcb59e2d129@redhat.com> Date: Thu, 24 Jul 2025 10:59:18 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/9] introduce PGTY_mgt_entry page_type To: Huan Yang , Andrew Morton , Lorenzo Stoakes , Rik van Riel , "Liam R. Howlett" , Vlastimil Babka , Harry Yoo , Xu Xin , Chengming Zhou , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , "Matthew Wilcox (Oracle)" , Christian Brauner , Usama Arif , Yu Zhao , Baolin Wang , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250724084441.380404-1-link@vivo.com> From: David Hildenbrand Content-Language: en-US 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/g1oFAmgsLPQFCRvGjuMACgkQTd4Q 9wD/g1o0bxAAqYC7gTyGj5rZwvy1VesF6YoQncH0yI79lvXUYOX+Nngko4v4dTlOQvrd/vhb 02e9FtpA1CxgwdgIPFKIuXvdSyXAp0xXuIuRPQYbgNriQFkaBlHe9mSf8O09J3SCVa/5ezKM OLW/OONSV/Fr2VI1wxAYj3/Rb+U6rpzqIQ3Uh/5Rjmla6pTl7Z9/o1zKlVOX1SxVGSrlXhqt kwdbjdj/csSzoAbUF/duDuhyEl11/xStm/lBMzVuf3ZhV5SSgLAflLBo4l6mR5RolpPv5wad GpYS/hm7HsmEA0PBAPNb5DvZQ7vNaX23FlgylSXyv72UVsObHsu6pT4sfoxvJ5nJxvzGi69U s1uryvlAfS6E+D5ULrV35taTwSpcBAh0/RqRbV0mTc57vvAoXofBDcs3Z30IReFS34QSpjvl Hxbe7itHGuuhEVM1qmq2U72ezOQ7MzADbwCtn+yGeISQqeFn9QMAZVAkXsc9Wp0SW/WQKb76 FkSRalBZcc2vXM0VqhFVzTb6iNqYXqVKyuPKwhBunhTt6XnIfhpRgqveCPNIasSX05VQR6/a OBHZX3seTikp7A1z9iZIsdtJxB88dGkpeMj6qJ5RLzUsPUVPodEcz1B5aTEbYK6428H8MeLq NFPwmknOlDzQNC6RND8Ez7YEhzqvw7263MojcmmPcLelYbfOwU0EVcufkQEQAOfX3n0g0fZz 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+DWgUCaCwtJQUJG8aPFAAKCRBN3hD3AP+DWlDnD/4k2TW+HyOOOePVm23F5HOhNNd7nNv3 Vq2cLcW1DteHUdxMO0X+zqrKDHI5hgnE/E2QH9jyV8mB8l/ndElobciaJcbl1cM43vVzPIWn 01vW62oxUNtEvzLLxGLPTrnMxWdZgxr7ACCWKUnMGE2E8eca0cT2pnIJoQRz242xqe/nYxBB /BAK+dsxHIfcQzl88G83oaO7vb7s/cWMYRKOg+WIgp0MJ8DO2IU5JmUtyJB+V3YzzM4cMic3 bNn8nHjTWw/9+QQ5vg3TXHZ5XMu9mtfw2La3bHJ6AybL0DvEkdGxk6YHqJVEukciLMWDWqQQ RtbBhqcprgUxipNvdn9KwNpGciM+hNtM9kf9gt0fjv79l/FiSw6KbCPX9b636GzgNy0Ev2UV m00EtcpRXXMlEpbP4V947ufWVK2Mz7RFUfU4+ETDd1scMQDHzrXItryHLZWhopPI4Z+ps0rB CQHfSpl+wG4XbJJu1D8/Ww3FsO42TMFrNr2/cmqwuUZ0a0uxrpkNYrsGjkEu7a+9MheyTzcm vyU2knz5/stkTN2LKz5REqOe24oRnypjpAfaoxRYXs+F8wml519InWlwCra49IUSxD1hXPxO WBe5lqcozu9LpNDH/brVSzHCSb7vjNGvvSVESDuoiHK8gNlf0v+epy5WYd7CGAgODPvDShGN g3eXuA== Organization: Red Hat In-Reply-To: <20250724084441.380404-1-link@vivo.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 24.07.25 10:44, Huan Yang wrote: > Summary > == > This patchset reuses page_type to store migrate entry count during the > period from migrate entry setup to removal, enabling accelerated VMA > traversal when removing migrate entries, following a similar principle to > early termination when folio is unmapped in try_to_migrate. I absolutely detest (ab)using page types for that, so no from my side unless I am missing something important. > > In my self-constructed test scenario, the migration time can be reduced How relevant is that in practice? > from over 150+ms to around 30+ms, achieving nearly a 70% performance > improvement. Additionally, the flame graph shows that the proportion of > remove_migration_ptes can be reduced from 80%+ to 60%+. > > Notice: migrate entry specifically refers to migrate PTE entry, as large > folio are not supported page type and 0 mapcount reuse. > > Principle > == > When a page removes all PTEs in try_to_migrate and sets up a migrate PTE > entry, we can determine whether the traversal of remaining VMAs can be > terminated early by checking if mapcount is zero. This optimization > helps improve performance during migration. > > However, when removing migrate PTE entries and setting up PTEs for the > destination folio in remove_migration_ptes, there is no such information > available to assist in deciding whether the traversal of remaining VMAs > can be ended early. Therefore, it is necessary to traversal all VMAs > associated with this folio. Yes, we don't know how many migration entries are still pointing at the page. > > In reality, when a folio is fully unmapped and before all migrate PTE > entries are removed, the mapcount will always be zero. Since page_type > and mapcount share a union, and referring to folio_mapcount, we can > reuse page_type to record the number of migrate PTE entries of the > current folio in the system as long as it's not a large folio. This > reuse does not affect calls to folio_mapcount, which will always return > zero. > > Therefore, we can set the folio's page_type to PGTY_mgt_entry when > try_to_migrate completes, the folio is already unmapped, and it's not a > large folio. The remaining 24 bits can then be used to record the number > of migrate PTE entries generated by try_to_migrate. In the future the page type will no longer overlay the mapcount and, consequently, be sticky. > > Then, in remove_migration_ptes, when the nr_mgt_entry count drops to > zero, we can terminate the VMA traversal early. > > It's important to note that we need to initialize the folio's page_type > to PGTY_mgt_entry and set the migrate entry count only while holding the > rmap walk lock.This is because during the lock period, we can prevent > new VMA fork (which would increase migrate entries) and VMA unmap > (which would decrease migrate entries). The more I read about PGTY_mgt_entry, the more I hate it. > > However, I doubt there is actually an additional critical section here, for > example anon: > > Process Parent fork > try_to_migrate > anon_vma_clone > write_lock > avc_inster_tree tail > .... > folio_lock_anon_vma_read copy_pte_range > vma_iter pte_lock > .... pte_present copy > ... > pte_lock > new forked pte clean > .... > remove_migration_ptes > rmap_walk_anon_lock > > If my understanding is correct and such a critical section exists, it > shouldn't cause any issues—newly added PTEs can still be properly > removed and converted into migrate entries. > > But in this: > > Process Parent fork > try_to_migrate > anon_vma_clone > write_lock > avc_inster_tree > .... > folio_lock_anon_vma_read copy_pte_range > vma_iter > pte_lock > migrate entry set > .... pte_lock > pte_nonpresent copy > .... > .... > remove_migration_ptes > rmap_walk_anon_lock Just a note: migration entries also apply to non-anon folios. -- Cheers, David / dhildenb