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[2003:cb:c731:4500:49a1:4659:3c82:9ce4]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4215810242fsm151646915e9.12.2024.06.09.13.28.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 09 Jun 2024 13:28:31 -0700 (PDT) Message-ID: <83de1b25-af25-4a68-8b35-d231338a2035@redhat.com> Date: Sun, 9 Jun 2024 22:28:30 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Unifying page table walkers To: Matthew Wilcox Cc: Khalid Aziz , Peter Xu , Vishal Moola , Jane Chu , Muchun Song , linux-mm@kvack.org References: <502bb09f-ea09-451b-8473-48b14dd2f554@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-Rspamd-Queue-Id: 3AEA7120010 X-Stat-Signature: d8y3363i9gkma67qffcisken6q3qbip6 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1717964916-435556 X-HE-Meta: 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 3GuqWYkv 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 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 09.06.24 22:08, Matthew Wilcox wrote: > On Fri, Jun 07, 2024 at 08:59:17AM +0200, David Hildenbrand wrote: >> On 06.06.24 20:29, Matthew Wilcox wrote: >>> One of the things we discussed at LSFMM was unifying the hugetlb and >>> THP page table walkers. I've been looking into it some more recently; >>> I've found a problem and I think a solution. >>> >>> The reason we have a separate hugetlb_entry from pmd_entry and pud_entry >>> is that it has a different locking context. It is called with the >>> hugetlb_vma_lock held for read (nb: this is not the same as the vma >>> lock; see walk_hugetlb_range()). Why do we need this? Because of page >>> table sharing. >>> >>> In a completely separate discussion, I was talking with Khalid about >>> mshare() support for hugetlbfs, and I suggested that we permit hugetlbfs >>> pages to be mapped by a VMA which does not have the VM_HUGETLB flag set. >>> If we do that, the page tables would not be permitted to be shared with >>> other users of that hugetlbfs file. But we want to eliminate support >>> for that anyway, so that's more of a feature than a bug. >> >> I am not sure why hugetlb support in mshare would require that (we don't >> need partial mappings and all of that to support mshare+hugetlb). > > You're absolutely right. My motivation is the other way around. A large > part of "hugetlbfs is special" is tied to the sharing of page tables. > That's why we have the hugetlb_vma_lock. If we're already sharing > page tables with mshare, I assert that it is not necessary to also > share page tables with other hugetlb users. So as part of including > hugetlb support in mshare, we should drop that support, and handle > hugetlb-mapped-with-mshare similarly to THP. Yes, we should absolutely *not* use hugetlb-page table sharing there! Regarding THP, I'm not sure how far we should go -- we should make our lives easier by not allowing partial mappings initially. > Possibly not the mapcount > parts so that we preserve the HVO. The new mapcount scheme I'll be reviving soon will not work easily with the existing hugetlb-page table sharing, simply because unrelated MMs can map/unmap pages in there, and we effectively transfer ownership of page tables between processes. As soon as we have one "mm" that owns one set of shared page tables (i.e., one mshare-mm that owns a set of shared page tables, and to which we effectively account the mappings in there), it should all just work. -- Cheers, David / dhildenb