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[2003:cb:c719:8200:487a:3426:a17e:d7b7]) by smtp.gmail.com with ESMTPSA id f12-20020a0565123b0c00b0051b65645256sm1221549lfv.265.2024.04.25.08.15.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Apr 2024 08:15:09 -0700 (PDT) Message-ID: Date: Thu, 25 Apr 2024 17:15:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to deferred split list To: Zi Yan Cc: linux-mm@kvack.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Yang Shi , Ryan Roberts , Barry Song <21cnbao@gmail.com>, Lance Yang , linux-kernel@vger.kernel.org References: <20240424224614.477221-1-zi.yan@sent.com> <8BBD1B75-135D-42AA-8937-53B259803AA7@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: <8BBD1B75-135D-42AA-8937-53B259803AA7@nvidia.com> X-Mimecast-Spam-Score: 0 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: rspam01 X-Rspamd-Queue-Id: 2DB8F80026 X-Stat-Signature: kucyouhii5b7zkad7qjba4p41phh13mt X-Rspam-User: X-HE-Tag: 1714058115-666889 X-HE-Meta: U2FsdGVkX1+EfEUFk+/NIuiH/iHp2lQ+ogZ3gtdnd/lVfgfGMJiHzNtgJgTngCHRMsSgkIcn7/nxXusXYHOSQkKXXBScbGIKwg7mQgOdd+vxtvHd/XtbrV+Ukoj/aTnkYt2/kWXfoGLqVOGoZ2tWIy1lSA8K3Nhrey4x3LskLPt2Pb4TRLoGBnD9VsEfslQyAd6Fa6bqcAlw7g4knTBZHXKQpaDZW6pe/X/TYloMNmEQnag8VfuHRTfT4gBbdUW2CeX5OYUEAZydYh8sd12LCMb5+Ltr5ReTR9Rv1bfK0kE6tiW4EGQGUfYgBeochv3UYjrdlBapTta+hSL1ILh9NoOd1B7TN0We8OYuzfklDQ6hBH7dtKylZycdWoXiNP2oIHFe91mUk6NmWD8AaeSeuPM9sl2OVXWYyMVoS78RWpR2LOs4cCQRDY73rN4XyMJxnqDZIncDJ4RLSwmVsQK53Arquj1+bALzY+tXXY4HeVwcmpmrnjLQilPHkbKe1x9dlw4agSj2/ERQNvPPgiyod0vVwkj2xqg9MdoYijaC1BclRtEnrzk8GX5RQVzOHfZ6Hr0NZiYpCShBi8qwg2Nxi3YNzu1f1YEL0GpH5JR/vE+BE9GmrAMTv3jrUP1aXsHrdqcKlfi64l6apuCUkiVXVwZsxCG8buU9r8KEY/S/FsxBonl42gJdY+ADEC1+5Oav9L/mZDuBVl489Xs+05awwBP5Go1csZRXLOMxdkT8xp1zDwdFunJfgeBFSsZjxmZjTCo1AVQX5Xw5yU7PTaFVU5sqAD1mAnr7bhKJpF+IErj5NKfMB3+fbHJInyE5G861+cttUIuNBvUI2Rr+FjCFMWAYkNj3LEkQ3IQ59l4plVUpQkl0Hjpakythr15RYVe+zD51WTAw97S1Pqpqew/XiWO2+A5MNRalTEVkbYaapMu6jbCHTVs3AIFjSP11zNjAar6HIcf1atZA57AdS30 TPeMwQQb ilak6m9BPC+ipSVXKe3hViQra9RhvoAaM3PnjkXsPJSHJyAmb4fqxrahOKs6bLMwQz/suD3aMvLtgj23ns4WIEpmlgH2vQdHoNvF/W5eg+f9TGjcMUkGL9BgJ/39Qy77WUav2Y55HaaKH/pyc3MOXAcx1Gv6Fwl9BqHlSQLh6QG2ycmmX2VXNve83KSM/J1DTi/FLmYrGJCYLSV7EdC6duP9i5HFRLqXkbVIwxBQD5glB06VdR35IsGCivgp85MFsiJEQecHDbzDMjXeq5HbqXNclSLefU2GYFdbByZbx6Xfgb3q+A3CWsKIZY8RRaKURr//awiNS+EP/ekfXwoJ5rtrlaev/9pZaP9eBqxrv7SntAUvH3oNDeCB9H5cZQ6R1n29huf9PzEthQrAQ1IIKCA3SUYNau1JQSjKNhy74mmX02CHDbI8L9d7EOK+wdx/0HrGxsoPSLF5prYOh+vwizc9/99yDDjPBR7Ft 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 25.04.24 16:53, Zi Yan wrote: > On 25 Apr 2024, at 3:19, David Hildenbrand wrote: > >> On 25.04.24 00:46, Zi Yan wrote: >>> From: Zi Yan >>> >>> In __folio_remove_rmap(), a large folio is added to deferred split list >>> if any page in a folio loses its final mapping. It is possible that >>> the folio is unmapped fully, but it is unnecessary to add the folio >>> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped >>> before adding a folio to deferred split list. If the folio is already >>> on the deferred split list, it will be skipped. This issue applies to >>> both PTE-mapped THP and mTHP. >>> >>> Commit 98046944a159 ("mm: huge_memory: add the missing >>> folio_test_pmd_mappable() for THP split statistics") tried to exclude >>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not >>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still >> >> Once again: your patch won't fix it either. >> >>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, >>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside >>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(). >>> However, this miscount was present even earlier due to implementation, >>> since PTEs are unmapped individually and first PTE unmapping adds the THP >>> into the deferred split list. >> >> It will still be present. Just less frequently. > > OK. Let me reread the email exchanges between you and Yang and clarify > the details in the commit log. Likely something like: -- In __folio_remove_rmap(), a large folio is added to deferred split list if any page in a folio loses its final mapping. But, it is possible that the folio is now fully unmapped and adding it to the deferred split list is unnecessary. For PMD-mapped THPs, that was not really an issue, because removing the last PMD mapping in the absence of PTE mappings would not have added the folio to the deferred split queue. However, for PTE-mapped THPs, which are now more prominent due to mTHP, we will always end up adding them to the deferred split queue. One side effect of this is that we will frequently increase the THP_DEFERRED_SPLIT_PAGE stat for PTE-mapped THP, making it look like we frequently get many partially mapped folios -- although we are simply unmapping the whole thing stepwise. Core-mm will now try batch-unmapping consecutive PTEs of PTE-mapped THPs where possible. If we're lucky, we unmap the whole thing in one go and can avoid adding the folio to the deferred split queue, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be noise when we cannot batch-unmap a complete PTE-mapped folio in one go -- or where this type of batching is not implemented yet. -- Feel free to reuse what you consider reasonable. -- Cheers, David / dhildenb