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[2003:cb:c743:7b00:745e:8663:4f85:318a]) by smtp.gmail.com with ESMTPSA id a16-20020a056000101000b003366a9cb0d1sm2762292wrx.92.2023.12.21.12.54.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 Dec 2023 12:54:03 -0800 (PST) Message-ID: Date: Thu, 21 Dec 2023 21:54:02 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 0/2] attempt to map anonymous pte-mapped THPs by pmds To: Zach O'Keefe , Xu Yu Cc: 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-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: BD4621A000A X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: dnayght9acg3c8fifzu67rg8udi1gmp9 X-HE-Tag: 1703192047-693872 X-HE-Meta: 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 VZPU4S1F 7nkYTwrMhyZUYlnU3Jm/VmsNaKY/ARiW3JNcWJzWVPejQrm0C61p6Xr5CuzrsjRWczt7r/pvxgTQaONUbb9ocI+QetCyt2tCiszgHEd1E6TkPVbBqG99URqzzmyr5rLNAz6PIIavodH7Mr76/mp0bfw8GElpXG/xUYA7L4MBaaN64MwpxvvhvZur6plME7lM4WpUOSYQYxmkLjQGh+18djEwJh+rLN6NpifZmxQuNdnECpN5GwclDjcHG3b5iDGMM5hOaNZfNnzqJ+RS0g2k6HVleyj/ZxF1nEFpGSGHu0MFHyMCoNODLjMvRbJ9SbT+QLSm2q8PvNWW7TiMj0P8aoHKzsWSEHm/mrWvfP/sEt3WqVkcOhHrJcMmORa5rIzECrQzPWa1Zq1VCbZ57X9tk5veyJeG+/0Ayyoq76NIks8t5qRU= 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 21.12.23 21:40, Zach O'Keefe wrote: > Hey Xu, > > Thanks for the patches. > > As a precursor, can you help understand what the use case is for these > patches? In-place collapse of anon memory is something I've thought > about before, but the opportunity has never been especially clear. > > In particular, your patches take an order-9 compound page, and just > try to see if we can update the mappings to it (like we do with > file/shmem). Functionally this seems fine, but the difference is that > with file/shmem, it's quite easy to have a pte-mapped-hugepage arise > naturally (the formation of the hugepage happening in the pagecache > being logically separate from the pmd-mapping of w/e task is mapping > it).\ > > For anonymous memory, the only time I can see us having a pte-mapped > hugepage (that isn't destined for splitting on deferred split list) > that we want to remap by a pmd is if we cause a VMA split + remerge by > mucking with VMA attributes. Yes, mostly because of madvise(), mprotect(), mremap(). But also, when putting a THP into the swap cache right now. When refaulting, you get a PTE-mapped THP. There are some other odd cases, and there might be more in the future (below) > > In my mind, what I had been thinking of w.r.t in-place anon collapse > was for the case where we've split a THP with MADV_FREE/MADV_DONTNEED > (i.e. to subrelease pages back to kernel), but later want to reform > the THP. In particular, if, for example, we only subrelease O(10s) of Right, and in-place collapse even works if the folio has been pinned, which is nice. > order-0 pages, it seems wasteful to have to reallocate a fresh > hugepage, then copy over O(100s) of pages, on collapse. If we were > able to attempt to first migrate-away any of those previously > subreleased pages (now possibly backing some other memory entirely), > it could save us from having to allocate a fresh order-9 page. Under > memory pressure / fragmentation, this could mean the difference > between success and failure. > One thing that popped up a couple of times already is that we might want to PTE-map a PMD-sized THP for a couple of reasons (IIRC, FreeBSD does some of that). For example: * Lazily zero the pages of the folio on demand, keeping all non-zeroed parts protnone. At a certain time (e.g., all zeroed), simply remap using a PMD. * Detecting sub-page access by temporarily mapping the THP using PTEs. Maybe, also some uffd optimizations, whereby protnone parts are not faulted in yet. -- Cheers, David / dhildenb