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b=fJn00l883ztTj0W6a4qKGBTVC4OuFVFH0jACQVcA/Ph1OECYYIL40jPNoxXYaSx+6 BoLGgJkVwTuw3r6m6XwSUBu4wXDID2H4TyJ8EBPplSRMPvdtR+rEzcLkuuerqdFqbV ZpV1+P4TKMLt6Ro8eYpYBV2abAfYDuI5wMjU2H9dlCT9yR7fmj7XJ83nDGO2S4yjB/ 87Zj81Psth2YSETQbC9odXcHDk9VMD+o77BMl/dlGFmB0V31dDl7Vz1kwhd9XleeCI DTfhEZAZvqaXK+IUAXo5KdFASU5skkR1+Tz5S9BIuii+YfkygptYY2GEp5G2/RpJsx QKbug7dh6Ii/A== Message-ID: <76c5576a-65d2-4dde-a969-ef6577855864@kernel.org> Date: Thu, 12 Mar 2026 20:58:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 0/6] Migrate on fault for device pages To: mpenttil@redhat.com, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Jason Gunthorpe , Leon Romanovsky , Alistair Popple , Balbir Singh , Zi Yan , Matthew Brost References: <20260211081301.2940672-1-mpenttil@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260211081301.2940672-1-mpenttil@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: EA67A20011 X-Stat-Signature: kf14p5m7mogrifd3ss9sgyi1ww8mbub5 X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1773345514-259180 X-HE-Meta: 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 JJezR9oU DoCJF32dIg4GWKrpAWgtqq8yTq0ia55di+Q7EMrOnUHh50E18CZIzrMitRlII3C/iUgGojlzaTbqP+nzfE8gGtLptCSLkjSu/3H/THd7Z+3PVoUIRQPvaR6aQUujdQyGHDXGJs3wAcUvHLZZw4b6jULxp8hB8OFHriubqH6jpbiCvxlOAjs2sJu9OzAHKrgGtnIjvM7nnxumLZa+3ePEH8A+Ez6AHylyrvXPrYp2mbXFnlKLrSgRi1fdp3xegDjGTNQlzdkBNRibYfjanr8NcgVo1q0PmA6FSKSq0jisv+3Obp6bqxqwKZebc1Cd2Nj01QzT3HH18a6tCRw6LSKryNGg0p13E1HKrL+8A Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/11/26 09:12, mpenttil@redhat.com wrote: > From: Mika Penttilä > > Currently, the way device page faulting and migration works > is not optimal, if you want to do both fault handling and > migration at once. > > Being able to migrate not present pages (or pages mapped with incorrect > permissions, eg. COW) to the GPU requires doing either of the > following sequences: > > 1. hmm_range_fault() - fault in non-present pages with correct permissions, etc. > 2. migrate_vma_*() - migrate the pages > > Or: > > 1. migrate_vma_*() - migrate present pages > 2. If non-present pages detected by migrate_vma_*(): > a) call hmm_range_fault() to fault pages in > b) call migrate_vma_*() again to migrate now present pages > > The problem with the first sequence is that you always have to do two > page walks even when most of the time the pages are present or zero page > mappings so the common case takes a performance hit. > > The second sequence is better for the common case, but far worse if > pages aren't present because now you have to walk the page tables three > times (once to find the page is not present, once so hmm_range_fault() > can find a non-present page to fault in and once again to setup the > migration). It is also tricky to code correctly. One page table walk > could costs over 1000 cpu cycles on X86-64, which is a significant hit. > > We should be able to walk the page table once, faulting > pages in as required and replacing them with migration entries if > requested. > > Add a new flag to HMM APIs, HMM_PFN_REQ_MIGRATE, > which tells to prepare for migration also during fault handling. > Also, for the migrate_vma_setup() call paths, a flag, MIGRATE_VMA_FAULT, > is added to tell to add fault handling to migrate. > > One extra benefit of migrating with hmm_range_fault() path > is the migrate_vma.vma gets populated, so no need to > retrieve that separataly. > > Tested in X86-64 VM with HMM test device, passing the selftests. > For performance, the migrate throughput tests from the selftests > show similar numbers (within error margin) as unmodified kernel. > Tested also rebased on the > "Remove device private pages from physical address space" series: > https://lore.kernel.org/linux-mm/20260130111050.53670-1-jniethe@nvidia.com/ > plus a small patch to adjust with no problems. Stumbling over this in my backlog, so far we have to RB tags. I'm going to take a look at the core MM bits (soon I hope), but it would be great if other people could review the HMM bits and provide proper tags. -- Cheers, David