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[2003:cb:c741:c700:3db9:94c9:28b0:34f2]) by smtp.gmail.com with ESMTPSA id u16-20020a05600c19d000b0041408e16e6bsm12099314wmq.25.2024.03.26.09.17.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Mar 2024 09:17:39 -0700 (PDT) Message-ID: <0ae22147-e1a1-4bcb-8a4c-f900f3f8c39e@redhat.com> Date: Tue, 26 Mar 2024 17:17:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64 To: Ryan Roberts , Mark Rutland , Catalin Marinas , Will Deacon , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Andrew Morton , Muchun Song Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240215121756.2734131-1-ryan.roberts@arm.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: <20240215121756.2734131-1-ryan.roberts@arm.com> 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-Server: rspam09 X-Rspamd-Queue-Id: A0C54C002A X-Stat-Signature: 9fusr1uqyjk7w8bsnf5g7ppb3jcurg8x X-Rspam-User: X-HE-Tag: 1711469866-842061 X-HE-Meta: U2FsdGVkX1+cWqM3t1BorC4F9jyQP2kcKfRkF+IGaHPGYoE8hD063G6HfuHc7NwJKq1hCa1d45QoiynlNdc+bqRBiqdShnFp+plHbVbklQfGT0LHU5wKmmTCtDAjknMABMWjSBB8NpiH7b7Dfv6hhUCLJ6ZqR2ykkNS2V1dntwXtodqpSu4VhNH+w4bM1GcfpFKko55RNXK54LnDqs3yCSE/Pso5RzfjvKan006hPVX2xRpON/TadNPpsCIG/87WxbBq4OBwpuLKQBVeZl+m3em0f/bO/j+bcV2BEAifuKQWN5MtCNyen/VZxPryWBNHGy8TA4trzEtCVAxR57sAN3vJXPFtZ5wyuJT4tIkK55FWSHHTMgNF7/VlhMEO1Tn0taxLmYUOjQoOdtMjILRjC+WfKbfF7cNoLu2SqoZ9+RLcxquMw1oZov2gK+sIuwQmpY5pm3cwFzeDp9PqQ6tywuxiPapHRp+4u8Pp24rjf31ElMH1bXajlRkwOlgEDUhxxzeulqXeLYOXjC+2V7GtTLr16s2D//ts7MtSeBssM/e1qsOxxTG3AFFFc2EX87EwGxcmQo2mLEI9iQAkrPIc5SkSbhWijX4cunF+aVGnX70JEyBlB7kdf8K7p+QHUcdwVYbzZbYUM8LG+YWj7nYHLly96sikrb3fLcGv0uodCyKpt0j8GBESDYPIu+7t0kukkklP0REPOlDDkUb0l0Yp6ltGtZ+djg8CFM8A9GI9zDLF5cnbcro59lb4llhUA413K5Xr9o8PPHgKcbiSAEBPOoR/4oarkHOEsb9TVAu2gBzPTbRFjn2Rrbpc91Prlar4XE6V521Poq0RCAInjNKEkwwiJpgdXFtYipdoMYggMaT5X5tjVS9ZgmH2cCvtTpjnHO1/5yhe81Pl9IXQkb7dIF0lVCWY+Ij1T7d0bK0UPf21LwE89Mm1ia1x012r4/c9tuLcUvg0SVElG0t4xkD 8/cYRllh JTy82VeZy/NjZF8n1hJYKgizznlrN/7hgie2Da+aEL94SrEG2kUZwS+PiM8dus8TXjiVXVkvmnGyaZLj72esoVUxU3p9OY6gMLrVbr0ifMjidZ0yorYiweb0SuBk+81MHBm8HBf7dhOhNHwT20oCHzY/+vjUmGnZGYIn4UYi543nCMlJLnoaC3MxECArRUQjPfgKkn9UuiLznhOg7ihtR2JoDJYd7Uq9oBFZXTWBUkgGznx8bjvile5MyJF5XKX3WpSkPJZMWw/8lcc5Gh1SHJXHnX/zDrsDUoCsnYG8zaXncgBqarhYfF1Mgqy0AQnLdtQNwprDqYi30Zghj+wGshOZ9RP5KUQ66KF7rs3Uk7dDk0VYcXe+2dyBLnN6f+E1QoBEjOwqCbhUq3MDiZugsp7Apn4V/xN1Y7rJDvxxYdNRUp4Wj48PlE7neiZdlwblpmJZctFZtPXlHtBA7vRlJ5FIqoQ== 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 15.02.24 13:17, Ryan Roberts wrote: > This is an RFC for a series that aims to reduce the cost and complexity of > ptep_get_lockless() for arm64 when supporting transparent contpte mappings [1]. > The approach came from discussion with Mark and David [2]. > > It introduces a new helper, ptep_get_lockless_norecency(), which allows the > access and dirty bits in the returned pte to be incorrect. This relaxation > permits arm64's implementation to just read the single target pte, and avoids > having to iterate over the full contpte block to gather the access and dirty > bits, for the contpte case. > > It turns out that none of the call sites using ptep_get_lockless() require > accurate access and dirty bit information, so we can also convert those sites. > Although a couple of places need care (see patches 2 and 3). > > Arguably patch 3 is a bit fragile, given the wide accessibility of > vmf->orig_pte. So it might make sense to drop this patch and stick to using > ptep_get_lockless() in the page fault path. I'm keen to hear opinions. Yes. Especially as we have these pte_same() checks that might just fail now because of wrong accessed/dirty bits? Likely, we just want to read "the real deal" on both sides of the pte_same() handling. > > I've chosen the name "recency" because it's shortish and somewhat descriptive, > and is alredy used in a couple of places to mean similar things (see mglru and > damon). I'm open to other names if anyone has better ideas. Not a native speaker; works for me. > > If concensus is that this approach is generally acceptable, I intend to create a > series in future to do a similar thing with ptep_get() -> ptep_get_norecency(). Yes, sounds good to me. -- Cheers, David / dhildenb