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Wed, 10 Jul 2024 17:15:40 -0700 (PDT) Received: from [10.35.209.243] ([208.115.86.71]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-70b4396841esm4376749b3a.132.2024.07.10.17.15.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Jul 2024 17:15:40 -0700 (PDT) Message-ID: <0f01c613-9e4f-47b6-af2b-09aa90437d90@redhat.com> Date: Thu, 11 Jul 2024 02:15:38 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 00/45] hugetlb pagewalk unification To: Oscar Salvador Cc: Peter Xu , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Muchun Song , SeongJae Park , Miaohe Lin , Michal Hocko , Matthew Wilcox , Christophe Leroy , Jason Gunthorpe , Ryan Roberts References: <20240704043132.28501-1-osalvador@suse.de> <617169bc-e18c-40fa-be3a-99c118a6d7fe@redhat.com> <84d4e799-90da-487e-adba-6174096283b5@redhat.com> <9d5980e3-72e6-4848-b1ac-83ffab8522c4@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-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: bx6utgru14dtnsuqeyhm44urmkyg1za7 X-Rspamd-Queue-Id: 4803F1C0028 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1720656944-907925 X-HE-Meta: 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 VQaZuEwA zqLLrnjcIcsLC71/g0b7l0ooZ7Jg9P46g5k3UYHHGbl1jyo8OuSTR9kbwGRGhLJ6V+/TNDPlnw/h8tXMp5+3pkj2n17OpKqmG4oO1rBhPgC3CjgWmF84SLUCnZrVsdesFhLf8qhD2o0NFglXRpehz0rTvQIfFZ919f7/vZvIhJwhXe+55Akg+kvYDmQouxFftRt2cy+y4zXkazHpWA8Uvrifp0YBMhCuOEYobQGN3cPF0Dn2dPp12eU0c8XL6LVZzw72cflC++PYMBE5N9FYHvzKFQSJm9f5zlDvKJaeUrYJUl7SaiwAF//a1iourAi/VnZAevClnIjJDfRvK74cMUinL7ZrMI69cVEP/gswQWd7GabYd4Tgy9Va2AD2vIShY/0Voidib2yvJsGcIveaKftZSvf7nLe610MeP/9BCS4ymVyA4CpSe7FtXBstr7fY3NzD6PX3ASKTSy/QEQ4BwYG3XD6pmntHChPQ76B+51lqyL+I= 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: >> (as a side note, cont-pte/cont-pmd should primarily be a hint from arch code >> on how many entries we can batch, like we do in folio_pte_batch(); point is >> that we want to batch also on architectures where we don't have such bits, >> and prepare for architectures that implement various sizes of batching; >> IMHO, having cont-pte/cont-pmd checks in common code is likely the wrong >> approach. Again, folio_pte_batch() is where we tackled the problem >> differently from the THP perspective) > > I must say I did not check folio_pte_batch() and I am totally ignorant > of what/how it does things. > I will have a look. > >> I have an idea for a better page table walker API that would try batching >> most entries (under one PTL), and walkers can just register for the types >> they want. Hoping I will find some time to at least scetch the user >> interface soon. >> >> That doesn't mean that this should block your work, but the >> cont-pte/cont/pmd hugetlb stuff is really nasty to handle here, and I don't >> particularly like where this is going. > > Ok, let me take a step back then. > Previous versions of that RFC did not handle cont-{pte-pmd} wide in the > open, so let me go back to the drawing board and come up with something > that does not fiddle with cont- stuff in that way. > > I might post here a small diff just to see if we are on the same page. > > As usual, thanks a lot for your comments David! Feel free to reach out to discuss ways forward. I think we should (a) move to the automatic cont-pte setting as done for THPs via set_ptes(). (b) Batching PTE updates at all relevant places, so we get no change in behavior: cont-pte bit will remain set. (c) Likely remove the use of cont-pte bits in hugetlb code for anything that is not a present folio (i.e., where automatic cont-pte bit setting would never set it). Migration entries might require thought (we can easily batch to achieve the same thing, but the behavior of hugetlb likely differs to the generic way of handling migration entries on multiple ptes: reference the folio vs. the respective subpages of the folio). Then we are essentially replacing what the current hugetlb_ functions do by the common way it is being done for THP (which does not exist for cont-pmd yet ... ). The only real alternative is special casing hugetlb all over the place to still call the hugetlb_* functions. :( it would all be easier without that hugetlb cont-pte/cont-pmd usage. CCing Ryan so he's aware. -- Cheers, David / dhildenb