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(p200300cbc70a03003ae1c3c00cef8413.dip0.t-ipconnect.de. [2003:cb:c70a:300:3ae1:c3c0:cef:8413]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c5c1b57b6sm15474346f8f.72.2025.02.04.02.56.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Feb 2025 02:56:09 -0800 (PST) Message-ID: <99f215f2-1e7f-4ff6-adec-e838916f6f4e@redhat.com> Date: Tue, 4 Feb 2025 11:56:07 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 04/12] mm/rmap: implement make_device_exclusive() using folio_walk instead of rmap walk To: Alistair Popple Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, nouveau@lists.freedesktop.org, Andrew Morton , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Jonathan Corbet , Alex Shi , Yanteng Si , Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pasha Tatashin , Peter Xu , Jason Gunthorpe References: <20250129115411.2077152-1-david@redhat.com> <20250129115411.2077152-5-david@redhat.com> <7tzcpx23vufmp5cxutnzhjgdj7kwqrw5drwochpv5ern7zknhj@h2s6y2qjbr3f> <180e9c2f-51fe-44ba-ac68-5aa7b7918ab0@redhat.com> From: David Hildenbrand Content-Language: en-US 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.01.25 23:31, Alistair Popple wrote: > On Thu, Jan 30, 2025 at 10:24:37AM +0100, David Hildenbrand wrote: >> On 30.01.25 10:01, David Hildenbrand wrote: >>> On 30.01.25 07:11, Alistair Popple wrote: >>>> On Wed, Jan 29, 2025 at 12:54:02PM +0100, David Hildenbrand wrote: >>>>> We require a writable PTE and only support anonymous folio: we can only >>>>> have exactly one PTE pointing at that page, which we can just lookup >>>>> using a folio walk, avoiding the rmap walk and the anon VMA lock. >>>>> >>>>> So let's stop doing an rmap walk and perform a folio walk instead, so we >>>>> can easily just modify a single PTE and avoid relying on rmap/mapcounts. >>>>> >>>>> We now effectively work on a single PTE instead of multiple PTEs of >>>>> a large folio, allowing for conversion of individual PTEs from >>>>> non-exclusive to device-exclusive -- note that the other way always >>>>> worked on single PTEs. >>>>> >>>>> We can drop the MMU_NOTIFY_EXCLUSIVE MMU notifier call and document why >>>>> that is not required: GUP will already take care of the >>>>> MMU_NOTIFY_EXCLUSIVE call if required (there is already a device-exclusive >>>>> entry) when not finding a present PTE and having to trigger a fault and >>>>> ending up in remove_device_exclusive_entry(). >>>> >>>> I will have to look at this a bit more closely tomorrow but this doesn't seem >>>> right to me. We may be transitioning from a present PTE (ie. a writable >>>> anonymous mapping) to a non-present PTE (ie. a device-exclusive entry) and >>>> therefore any secondary processors (eg. other GPUs, iommus, etc.) will need to >>>> update their copies of the PTE. So I think the notifier call is needed. >>> >>> Then it is all very confusing: > > Can't argue with that in hindsight :-) > >>> "MMU_NOTIFY_EXCLUSIVE: to signal a device driver that the device will no >>> longer have exclusive access to the page." >> >> So the second sentence actually describes the other condition. Likely we >> should make that clearer: >> >> --- a/include/linux/mmu_notifier.h >> +++ b/include/linux/mmu_notifier.h >> @@ -43,10 +43,11 @@ struct mmu_interval_notifier; >> * a device driver to possibly ignore the invalidation if the >> * owner field matches the driver's device private pgmap owner. >> * >> - * @MMU_NOTIFY_EXCLUSIVE: to signal a device driver that the device will no >> - * longer have exclusive access to the page. When sent during creation of an >> - * exclusive range the owner will be initialised to the value provided by the >> - * caller of make_device_exclusive(), otherwise the owner will be NULL. >> + * @MMU_NOTIFY_EXCLUSIVE: (1) to signal a device driver that the device will no >> + * longer have exclusive access to the page; and (2) to signal that a page will >> + * be made exclusive to a device. During (1), the owner will be NULL, during >> + * (2), the owner will be initialised to the value provided by the caller of >> + * make_device_exclusive(). > > Yes, I think that makes things clearer. Logically these are really two different > events though - I guess I didn't want to add another one at the time but I > wonder if we should just make them separate events rather than overloading them? I had the same thought and then I wondered: can't we simply use MMU_NOTIFY_CLEAR for the exclusive->ordinary path? I mean, it's essentially a zap+flush followed by a re-insertion of the PFN swap entry. Similar to page migration ... -- Cheers, David / dhildenb