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(p200300d82f4d1a004fdf53e21a36ba34.dip0.t-ipconnect.de. [2003:d8:2f4d:1a00:4fdf:53e2:1a36:ba34]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a1f58f2af7sm17091148f8f.52.2025.05.13.10.17.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 13 May 2025 10:17:12 -0700 (PDT) Message-ID: <9fc4ad12-869c-492d-898c-9368505199d0@redhat.com> Date: Tue, 13 May 2025 19:17:10 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 00/11] mm: rewrite pfnmap tracking and remove VM_PAT To: "Liam R. Howlett" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-trace-kernel@vger.kernel.org, Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , David Airlie , Simona Vetter , Andrew Morton , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pedro Falcato , Peter Xu References: <20250512123424.637989-1-david@redhat.com> <2smxfvgmkrac4uzjwti5sgv2ubzsfgt24e6yinwtl7iuchxjt3@c2ejownpooio> 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: <2smxfvgmkrac4uzjwti5sgv2ubzsfgt24e6yinwtl7iuchxjt3@c2ejownpooio> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: dBQWYuuYqiPYvIOx4fJnoVk_KFGwdsZkSuWOxvJmAbE_1747156633 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 13.05.25 17:53, Liam R. Howlett wrote: > * David Hildenbrand [250512 08:34]: >> On top of mm-unstable. >> >> VM_PAT annoyed me too much and wasted too much of my time, let's clean >> PAT handling up and remove VM_PAT. >> >> This should sort out various issues with VM_PAT we discovered recently, >> and will hopefully make the whole code more stable and easier to maintain. >> >> In essence: we stop letting PAT mode mess with VMAs and instead lift >> what to track/untrack to the MM core. We remember per VMA which pfn range >> we tracked in a new struct we attach to a VMA (we have space without >> exceeding 192 bytes), use a kref to share it among VMAs during >> split/mremap/fork, and automatically untrack once the kref drops to 0. > > What you do here seems to be decouple the vma start/end addresses by > abstracting them into another allocated ref counted struct. This is > close to what we do with the anon vma name.. Yes, inspired by that. > > It took a while to understand the underlying interval tree tracking of > this change, but I think it's as good as it was. IIRC, there was a > shrinking and matching to the end address in the interval tree, but I > failed to find that commit and code - maybe it never made it upstream. > I was able to find a thread about splitting [1], so maybe I'm mistaken. There was hidden code that kept a memremap() shrinking working (adjusting the tracked range). The leftovers are removed in patch #8. See below. > >> >> This implies that we'll keep tracking a full pfn range even after partially >> unmapping it, until fully unmapping it; but as that case was mostly broken >> before, this at least makes it work in a way that is least intrusive to >> VMA handling. >> >> Shrinking with mremap() used to work in a hacky way, now we'll similarly >> keep the original pfn range tacked even after this form of partial unmap. >> Does anybody care about that? Unlikely. If we run into issues, we could >> likely handled that (adjust the tracking) when our kref drops to 1 while >> freeing a VMA. But it adds more complexity, so avoid that for now. > > The decoupling of the vma and ref counted range means that we could beef > up the backend to support actually tracking the correct range, which > would be nice.. Right, in patch #4 I have " This change implies that we'll keep tracking the original PFN range even after splitting + partially unmapping it: not too bad, because it was not working reliably before. The only thing that kind-of worked before was shrinking such a mapping using mremap(): we managed to adjust the reservation in a hacky way, now we won't adjust the reservation but leave it around until all involved VMAs are gone. If that ever turns out to be an issue, we could hook into VM splitting code and split the tracking; however, that adds complexity that might not be required, so we'll keep it simple for now. " Duplicating/moving/forking VMAs is now definitely better than before. Splitting is also arguably better than before -- even a simple partial munmap() [1] is currently problematic, unless we're munmapping the last part of a VMA (-> shrinking). Implementing splitting properly is a bit complicated if the pnfmap ctx has more than one ref, but it could be added if ever really required. [1] https://lkml.kernel.org/r/20250509153033.952746-1-david@redhat.com > but I have very little desire to work on that. Jap :) > > > [1] https://lore.kernel.org/all/5jrd43vusvcchpk2x6mouighkfhamjpaya5fu2cvikzaieg5pq@wqccwmjs4ian/ > >> >> Briefly tested with the new pfnmap selftests [1]. >> >> [1] https://lkml.kernel.org/r/20250509153033.952746-1-david@redhat.com > > oh yes, that's still a pr_info() log. I think that should be a pr_err() > at least? I was wondering if that is actually a WARN_ON_ONCE(). Now, it should be much harder to actually trigger. Thanks! -- Cheers, David / dhildenb