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[2003:cb:c713:3b00:16ce:8f1c:dd50:90fb]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438e23dea58sm36787055e9.15.2025.01.30.15.19.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Jan 2025 15:19:26 -0800 (PST) Message-ID: <4c50a439-e2b8-4f54-ba3d-366d0e2961b2@redhat.com> Date: Fri, 31 Jan 2025 00:19:24 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] HugeTLB generic pagewalk To: Oscar Salvador , lsf-pc@lists.linux-foundation.org Cc: Peter Xu , Muchun Song , 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: 8ORT48S8oci_VZYvdi8e7dh5XZ480I7Axc-KzsHEnrI_1738279168 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: CCADEC0006 X-Stat-Signature: qrskks3g458nw8cxxd3gjmqp1ofemwaa X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1738279171-516182 X-HE-Meta: U2FsdGVkX19ggGZIbsfSgbG+eeXotAmIX0RWKQfnuD/7W645WOp/K5NjSlWKR63ucvBE4GFcpz3GgOpLIaUhOIBgudBHi2S4chk6/4459TG08w6JDgW78PgEXcYDoAHSYIxyZc79sCWl7MFFXV0SpiXwInNVbFElwlGEmcg/2Dbdmk90Tmv943qXucdbykDmlY+JfOTXb2tu2wUcsrWQAV+5vo22GUMiDnGEuwNscXy4v/sfNfdPXF0k8Bsp/Fh9I+pmKhRlS4XNkAqblO8uop5QWiE99cfdgbgcqmwQzZUZGjGTUHBaYfcQtVX0TOLEpldTVhmDXSdyo8DF3o1/Dsfck+7vnCzcAmFDUaLvZCMAL7ty2ADJ02R7eTq9xHBjlB3CoatvbbSWoORUmyEgLtMtH9n1pEW8PsmOpgArM3XbeOuiz6zPCg4V7y85PlC37Jg6OAqMyRomJab6MYjDtB9Tm4K/np7641EOlexNgCbZQ1PXjtR4dVjQm9x1cl7IiBKe9mjFP2gbjYtZU2UQel938eucgZmG7/V2zOc0nkmf90F6afCGcY2rDUeR6G0Tc2zcM+1JafPIJE0xXZ6sdk8liA0ST6uQYgfa5ASGa2Ax118guls4dbx77fLSqsxnzNvmldTagJ0Dx9rSU97PCAO0eOiHzt1ZC20TU6Nj0kkOD36Ec8rAjwL4RBiGqZlcsAVvLveM8C0xjAkO5QzQ85NqdkJktvxximHJ29oo24gFmJSVzW8xzpxCyDoopbKe9EHUn/jXHFFjKqDoYrvWkeqQtbjXjgNX4d13R3xW9zNpIlMBMMqeT4hHqxCAf8GE1L65veRZwjpbMZu4Wa0o7nxoLIBCJ/kDrgU/NywKqug8+ePSlLUK+yW7pTQ0ylektfIkkaeSA77xU5tcDUcYaTgc+zmsrV2mcQOW8zQRVi5VIzKM36V2FkAv/MDRpQgLVeTxwUkwvBsWgTBoIhq UwuDlfGV qQx9k+i/IBxIBj66ScCUYT2ChzEJB2zItbSNfy+/502aP/0eBCA5/dlrLOYlUK+xyluwtkQo5cDFZOIu5La5r7MmFHYUrMhy3sNLjcQiJF+f6+k1gO6yI7FSu2Nm9hl1A4D23RjmXLLb/ohcjcAVLwgSELWMIvtgVUeceeYTw0uvzq2RSf9iWOsYMld2cXNKhI6nZ63jjonH4+ZZGaNtrpXHuKOwGr1MPRYW4n+9Y88TBKt6STAKdReRFkJVHbHXHcLxqJa1ow+u6L3GPWBSpSjE3D+Wky7QUvL2zqEkFq8KH63jUdXqerey0QEi7I7E4af0UyhlZ2BsI3ybnLcJ/f0+9KErrgyV0roBuex1lfb3Fkx1yTUx3egNdS4eVsOrdE9hgWK4/4dBjbNWD/tDN3WZsEyJyM94UHUMPwO4Uwv3h6JpueyXpYHWh93Qqq+1LfkBLKxngxOtdMwg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000006, 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 30.01.25 22:36, Oscar Salvador wrote: > Hi, > > last year Peter Xu did a presention at LSFM/MM on how to better integrate hugetlb > in the mm core. > There are several reasons we want to do that, but one could say that the two that > matter the most are 1) code duplication and 2) making hugetlb less special. > > During the last year several patches that went in that direction were merged e.g: > gup hugetlb unify [1], mprotect for dax PUDs [2], hugetlb into generic unmapping > path [3] to name some. > > There was also a concern on how to integrate hugetlb into the generic pagewalk, > getting rid by doing so of a lot of code and have a generic path that could handle > everything. > This was first worked in [4] (very basic draft). > > Although a second version is on the works, I would like to present some concerns > I have wrt. that work. Hi Oscar, > > HugeTLB has its own way of dealing with things. > E.g: HugeTLB interprets everything as a pte: huge_pte_uffd_wp, huge_pte_clear_uffd_wp, > huge_pte_dirty, huge_pte_modify, huge_pte_wrprotect etc. > > One of the challenges that this raises is that if we want pmd/pud walkers to > be able to make sense of hugetlb stuff, we need to implement pud/pmd > (maybe some pmd we already have because of THP) variants of those. that's the easy case I'm afraid. The real problem are cont-pte constructs (or worse) abstracted by hugetlb to be a single unit ("hugetlb pte"). For "ordinary" pages, the cont-pte bit (as on arm64) is nowadays transparently managed: you can modify any PTE part of the cont-gang and it will just work as expected, transparently. Not so with hugetlb, where you have to modify (or even query) the whole thing. For GUP it was easier, because it was able to grab all information it needed from the sub-ptes fairly easily, and it doesn't modify any page tabls. I ran into this problem with folio_walk, and had to document it rather nastily: * WARNING: Modifying page table entries in hugetlb VMAs requires a lot of care. * For example, PMD page table sharing might require prior unsharing. Also, * logical hugetlb entries might span multiple physical page table entries, * which *must* be modified in a single operation (set_huge_pte_at(), * huge_ptep_set_*, ...). Note that the page table entry stored in @fw might * not correspond to the first physical entry of a logical hugetlb entry. I wanted to use it to rewrite the uprobe code to also handle hugetlb with less special casing, but that work stalled so far. I think my next attempt would rule out any non-pmd / non-pud hugetlb pages to make it somewhat simpler. It all gets weird with things like: commit 0549e76663730235a10395a7af7ad3d3ce6e2402 Author: Christophe Leroy Date: Tue Jul 2 15:51:25 2024 +0200 powerpc/8xx: rework support for 8M pages using contiguous PTE entries In order to fit better with standard Linux page tables layout, add support for 8M pages using contiguous PTE entries in a standard page table. Page tables will then be populated with 1024 similar entries and two PMD entries will point to that page table. The PMD entries also get a flag to tell it is addressing an 8M page, this is required for the HW tablewalk assistance. Where we are walking a PTE table, but actually there is another PTE table we have to modify in the same go. Very hard to make that non-hugetlb aware, as it's simply completely different compared to ordinary page table walking/modifications today. Maybe there are ideas to tackle that, and I'd be very interested in them. -- Cheers, David / dhildenb