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(p200300cbc7133b0016ce8f1cdd5090fb.dip0.t-ipconnect.de. [2003:cb:c713:3b00:16ce:8f1c:dd50:90fb]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438e23de772sm20187955e9.13.2025.01.30.03.42.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Jan 2025 03:42:28 -0800 (PST) Message-ID: <887df26d-b8bb-48df-af2f-21b220ef22e6@redhat.com> Date: Thu, 30 Jan 2025 12:42:26 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 12/12] mm/rmap: keep mapcount untouched for device-exclusive entries To: 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 , Alistair Popple , Jason Gunthorpe References: <20250129115411.2077152-1-david@redhat.com> <20250129115411.2077152-13-david@redhat.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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: OwTIMRC_n3sHDcS2orQs90vtscNNDWItzSO45hlhrOg_1738237349 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: 4A8EB100007 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: sifxsoiy54r36wwqka3qruxqgcqyffz6 X-HE-Tag: 1738237352-136859 X-HE-Meta: U2FsdGVkX1+7MAfGw7PFEJjceNog3vgNrwHL4J+rerzDcoFCNnbY0Uwt+9pdBlDpI5ibnZjBr+zRHjRdHp7hTvYPoBWOP+GJO5ONIpJGhS3M+SHrJ6hnIEsz3AK5jgPTM98OtEseJQF9qf0UQJ8Qn3dwKQ+cAtE7P+TO84raTrqWlWMoGkNya1utquy2Y7VKnFaKjqveATkHcyidGySYF7/V8V0swmSxKs88h7Ee+KXA6oOFt2bMIkk0QTXoEVC+oOzh3rrcZHmdWHDgiuV6qLh6h3ljhX3o09r/xKGGRJQ4GZu5cvppikDfl5fdq+tp5DTsW+XnKkGzTe0zjuFZBaUOCP2JYn07Voy/uywctPrFo2UaoQJKJ7KN51Ne1VKTBo9bdUAo9G/NPwDDwf5yBJv30KRYEKaSbDf8YJsEsBomzqW0dlyA7Gar5p9EDBzb5sc/XHKq87e7ral+HdobIvm5rHpACM5nGdn05k6/HjDaAvMALHS+LvJQfa/GVRPkRIHqGXHwbTadMs2ZiZbKT1s5x4/i8HtD+CrLwjIfW9aBM5WH7sF/nWfpvwkwawLJFM/eFqW/QMK4D1fh/bR7XuTsxuzrwU3vpBvwRvhZ8c31uiD//4XB3f4LGAVk9m1CumGtNBcSsxoaLzly5qZ4B+Sjat5nthe/jARXWN2oh5PEDKgmd8T41OOkWbCCGwtToIEXvH23UPkPFL3gatb0bxqtSw4qU5FlLxZQwSlWxwGrm9Z6ELRhTEmnw9P6FL98GiDNBd2DbIEiarC5jtLmK0auUaRpt/12PQ/qr3tx1itW7ElJ17E8fUq5HrbXdrdwHrJkUzdj9sySq2Zl6xzCyQt4VwaQVUfsP3a5PcIsaog05ENuIG46TLS9Qcsx/BRk/5vUqaVeRtLj4DoNTR2HcUxijYxyodM4FBh/tsZvWPxY0ZMu1pmcnHnIgbQIghXyLzjqQreW9nAtseQvM6i brtZy02H w3UiwvQNwSGXjXJleWWBQs7Om0MlzM8xbHGQrIKnWiv0plDl9hx7pvOr/HoIRTUeq2zJ32B5mrzb3t7e8c2hcgKdt57P7BGpRxvr0nWAftxtGYJXZM8W7LASIEo1v9C8LWk7R8hOqepVf3eGBYJDYYxYOM1JL96G5RKFtGy/wOxE4yyhILBCFs5WfQ1KZML+aeQg4QwqnJm3UbnD5YYylOO3c24HTqXAhqjluK8zMfFXxGROYD3ygJ7HhxebzhuKHOVSChq4LM7IIEmxgGqvw3rRBQEz9sDoPXqyyeUB+9l1nrw/1wfqM0Hthqa1oRH5gqF7hbrP5B4+iWkZBCR9HEBDeFrlxg2BCv8wXPNGaCfx21oyqK4p5aF3JZytiEL0/kkd9U/Oh7k7rt/w= 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 30.01.25 11:37, Simona Vetter wrote: > On Wed, Jan 29, 2025 at 12:54:10PM +0100, David Hildenbrand wrote: >> Now that conversion to device-exclusive does no longer perform an >> rmap walk and the main page_vma_mapped_walk() users were taught to >> properly handle nonswap entries, let's treat device-exclusive entries just >> as if they would be present, similar to how we handle device-private >> entries already. > > So the reason for handling device-private entries in rmap is so that > drivers can rely on try_to_migrate and related code to invalidate all the > various ptes even for device private memory. Otherwise no one should hit > this path, at least if my understanding is correct. Right, device-private probably only happen to be seen on the migration path so far. > > So I'm very much worried about opening a can of worms here because I think > this adds a genuine new case to all the various callers. To be clear: it can all already happen. Assume you have a THP (or any mTHP today). You can easily trigger the scenario that folio_mapcount() != 0 with active device-exclusive entries, and you start doing rmap walks and stumble over these device-exclusive entries and *not* handle them properly. Note that more and more systems are configured to just give you THP unless you explicitly opted-out using MADV_NOHUGEPAGE early. Note that b756a3b5e7ea added that hunk that still walks these device-exclusive entries in rmap code, but didn't actually update the rmap walkers: @@ -102,7 +104,8 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw) /* Handle un-addressable ZONE_DEVICE memory */ entry = pte_to_swp_entry(*pvmw->pte); - if (!is_device_private_entry(entry)) + if (!is_device_private_entry(entry) && + !is_device_exclusive_entry(entry)) return false; pfn = swp_offset(entry); That was the right thing to do, because they resemble PROT_NONE entries and not migration entries or anything else that doesn't hold a folio reference). Fortunately, it's only the page_vma_mapped_walk() callers that need care. mm/rmap.c is handled with this series. mm/page_vma_mapped.c should work already. mm/migrate.c: does not apply mm/page_idle.c: likely should just skip !pte_present(). mm/ksm.c might be fine, but likely we should just reject !pte_present(). kernel/events/uprobes.c likely should reject !pte_present(). mm/damon/paddr.c likely should reject !pte_present(). I briefly though about a flag to indicate if a page_vma_mapped_walk() supports these non-present entries, but likely just fixing them up is easier+cleaner. Now that I looked at all, I might just write patches for them. > >> This fixes swapout/migration of folios with device-exclusive entries. >> >> Likely there are still some page_vma_mapped_walk() callers that are not >> fully prepared for these entries, and where we simply want to refuse >> !pte_present() entries. They have to be fixed independently; the ones in >> mm/rmap.c are prepared. > > The other worry is that maybe breaking migration is a feature, at least in > parts. Maybe breaking swap and migration is a feature in some reality, in this reality it's a BUG :) If thp constantly reassembles a pmd entry because hey all the > memory is contig and userspace allocated a chunk of memory to place > atomics that alternate between cpu and gpu nicely separated by 4k pages, > then we'll thrash around invalidating ptes to no end. So might be more > fallout here. khugepaged will back off once it sees an exclusive entry, so collapsing could only happen once everything is non-exclusive. See __collapse_huge_page_isolate() as an example. It's really only page_vma_mapped_walk() callers that are affected by this change, not any other page table walkers. It's unfortunate that we now have to fix it all up, that original series should have never been merged that way. -- Cheers, David / dhildenb