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([2a01:599:904:96e0:a245:aa9f:6c57:eb41]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c5c121951sm1419530f8f.45.2025.01.30.01.37.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Jan 2025 01:37:08 -0800 (PST) Message-ID: Date: Thu, 30 Jan 2025 10:37:06 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 4/4] mm/memory: document restore_exclusive_pte() 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: <20250129115803.2084769-1-david@redhat.com> <20250129115803.2084769-5-david@redhat.com> <7vejbjs7btkof4iguvn3nqvozxqpnzbymxbumd7pant4zi4ac4@3ozuzfzsm5tp> 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: <7vejbjs7btkof4iguvn3nqvozxqpnzbymxbumd7pant4zi4ac4@3ozuzfzsm5tp> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.01.25 01:27, Alistair Popple wrote: > On Wed, Jan 29, 2025 at 12:58:02PM +0100, David Hildenbrand wrote: >> Let's document how this function is to be used, and why the requirement >> for the folio lock might maybe be dropped in the future. > > Sorry, only just catching up on your other thread. The folio lock was to ensure > the GPU got a chance to make forward progress by mapping the page. Without it > the CPU could immediately invalidate the entry before the GPU had a chance to > retry the fault. > > Obviously performance wise having such thrashing is terrible, so should > really be avoided by userspace, but the lock at least allowed such programs > to complete. Thanks for the clarification. So it's relevant that the MMU notifier in remove_device_exclusive_entry() is sent after taking the folio lock. However, as soon as we drop the folio lock, remove_device_exclusive_entry() will become active, lock the folio and trigger the MMU notifier. So the time it is actually mapped into the device is rather > >> Signed-off-by: David Hildenbrand >> --- >> mm/memory.c | 25 +++++++++++++++++++++++++ >> 1 file changed, 25 insertions(+) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 46956994aaff..caaae8df11a9 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -718,6 +718,31 @@ struct folio *vm_normal_folio_pmd(struct vm_area_struct *vma, >> } >> #endif >> >> +/** >> + * restore_exclusive_pte - Restore a device-exclusive entry >> + * @vma: VMA covering @address >> + * @folio: the mapped folio >> + * @page: the mapped folio page >> + * @address: the virtual address >> + * @ptep: PTE pointer into the locked page table mapping the folio page >> + * @orig_pte: PTE value at @ptep >> + * >> + * Restore a device-exclusive non-swap entry to an ordinary present PTE. >> + * >> + * The folio and the page table must be locked, and MMU notifiers must have >> + * been called to invalidate any (exclusive) device mappings. In case of >> + * fork(), MMU_NOTIFY_PROTECTION_PAGE is triggered, and in case of a page >> + * fault MMU_NOTIFY_EXCLUSIVE is triggered. >> + * >> + * Locking the folio makes sure that anybody who just converted the PTE to >> + * a device-private entry can map it into the device, before unlocking it; so >> + * the folio lock prevents concurrent conversion to device-exclusive. > > I don't quite follow this - a concurrent conversion would already fail > because the GUP in make_device_exclusive_range() would most likely cause > an unexpected reference during the migration. And if a migration entry > has already been installed for the device private PTE conversion then > make_device_exclusive_range() will skip it as a non-present entry anyway. Sorry, I meant "device-exclusive", so migration is not a concern. > > However s/device-private/device-exclusive/ makes sense - the intent was to allow > the device to map it before a call to restore_exclusive_pte() (ie. a CPU fault) > could convert it back to a normal PTE. > >> + * TODO: the folio lock does not protect against all cases of concurrent >> + * page table modifications (e.g., MADV_DONTNEED, mprotect), so device drivers >> + * must already use MMU notifiers to sync against any concurrent changes > > Right. It's expected drivers are using MMU notifiers to keep page tables in > sync, same as for hmm_range_fault(). Let me try to rephrase it given that the folio lock is purely to guarantee forward-progress, not for correctness; that's what MMU notifiers must be used for. -- Cheers, David / dhildenb