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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4391dca34a8sm175720725e9.16.2025.02.11.05.31.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 11 Feb 2025 05:32:00 -0800 (PST) Message-ID: Date: Tue, 11 Feb 2025 14:31:58 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] HugeTLB generic pagewalk To: Oscar Salvador Cc: lsf-pc@lists.linux-foundation.org, Peter Xu , Muchun Song , linux-mm@kvack.org References: <4c50a439-e2b8-4f54-ba3d-366d0e2961b2@redhat.com> <74ecaa8b-9e94-4ba8-a2f0-a312607516ba@redhat.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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: -nNA0UjOBMvheQxUhVKAyAc-kEhYP2O1MsI1fxgIBrE_1739280722 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: E455714000B X-Stat-Signature: 8w96ztgowhae5ies4fuixusgymr5xh1x X-Rspamd-Server: rspam03 X-HE-Tag: 1739280724-232387 X-HE-Meta: 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 9pQj7Nzb tJNv5AGqr+Ad50rn72/CLYMInVTADMVdCCinyImBbKv3J2rZJAH57Mp5yTZT+59JJd/Y45tnKW7Y8I9avNEQhK+3BgOM4hR0mQPv/Bp8wrRkNgYvycYJPotUH9JcnPKsbUtP1Ki697au7AUfZbYO21e8dPSLXxMECPE1BbTAFh9kQ9svtv1lpKBCyGLfQLLspUwV0I3qmGpvl3bq62wybyQQg+4SMYfoS+hQxgHtq1XIpeufy+EKknTP0ipkcxoG3MLz7TWCJib1PWKew6lP2sB/3TTHRnfB+LRLvGghD04XHlYhxKam94R/Pj/0f7qExH40JP/8R3i1wXLAgUfNXGS4+BNO8eV2JnxSIm9Q7+BqaMbhDcr6RJDf3snvXxWew3Ng5/CXsofP+tBX2OA5PVHXUZTPZEPSNM06L37mUGC6xqeheWiW2PEBp5wibLqtXVdYcgrD6qnnFOdw= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000561, 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 05.02.25 10:33, Oscar Salvador wrote: > On Tue, Feb 04, 2025 at 09:40:16PM +0100, David Hildenbrand wrote: >> Unfortunately not that easy. You can only have parts of a large folio >> mapped. It would be PAGE_SIZE * nr_ptes when batching. > > I see, you are right. > >> I think this is the wrong approach. We should replace that pagewalk API >> usage by something better that hides all the batching. >> >> The interface would look similar to folio_walk (no callbacks, handling of >> locking), but (a) work on ranges; (b) work also on non-folio entries; and >> (c) batch all suitable entries in the range. >> >> Something like a pt_range_walk_start() that returns a "type" (folio range, >> migration entries, swap range, ...) + stores other details (range, level, >> ptep, ...) in a structure like folio_walk, to then provide mechanisms to >> continue (pt_walk_continue()) to walk or abort (pt_walk_done()) it. Similar >> to page_vma_mapped_walk(), but not specific to a given page/folio. >> >> Then, we would simply process the output of that. With the hope that, for >> hugetlb it will just batch all cont-pte / cont-pmd entries into a single >> return value. >> >> That will make the R/O walking as in task_mmu.c easier, hopefully. >> >> Not so much with PTE/PMD modifications, like damon_mkold_ops ... :( But >> maybe that just has to be special-cased for hugetlb, somehow ... > > Ok, let me see if we are on the same page. > You are basically saying that we should replace the existing pagewalk API with > something similar to what you described above. Yes, something that adds a better abstraction and walks ranges (e.g., folio range, swap range, ..) moving away from the per-pte callbacks etc. > > I have to confess that when you first mentioned this back in July when I > posted the RFC, I felt dishearted, because it implies an even bigger > surgery. Yeah. But it will be worth it in the long run (at least I hope so ;) ) > > But having felt the mess that dealing with cont-{pmd,pud}s, and the > inability of the existing API to do that in a clean way (without having > to teach each and every function about that if needed), maybe it is the > only way to do this 1) right and 2) clean. Jup > I thought that maybe we can get away and to the batching somehow before > calling in the callbacks e.g: at walk_{pud,pmd,pte}_range level, but I > am not sure whether 1) that is possible and 2) how ugly it would look. Exactly. > > So, given that this is not a really urgent matter, something that needs > to be fixed asap, maybe the way to go is to create an API that can deal > with all that, abstracting all these details. Yes, it's been on my todo list for longer (after I completed folio_walk), but I didn't get to it yet. > > I am willing to take a shot on this, if we are clear that it makes sense > to pursue this road. Yes, feel free to use what I propose as a starting point. If I get to prototype something in the meantime, I'll send it to you. (staring at my inbox, I'm not so sure I'll get to it shortly :) ) -- Cheers, David / dhildenb