From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id C7661C021A4 for ; Wed, 12 Feb 2025 19:02:12 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 3EBA86B007B; Wed, 12 Feb 2025 14:02:12 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 39BDF6B0082; Wed, 12 Feb 2025 14:02:12 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 215F96B0083; Wed, 12 Feb 2025 14:02:12 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id EE4806B007B for ; Wed, 12 Feb 2025 14:02:11 -0500 (EST) Received: from smtpin12.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 63B3549C7B for ; Wed, 12 Feb 2025 19:02:11 +0000 (UTC) X-FDA: 83112212862.12.8A172A3 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf28.hostedemail.com (Postfix) with ESMTP id 708AAC001B for ; Wed, 12 Feb 2025 19:02:06 +0000 (UTC) Authentication-Results: imf28.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=ZnibefHr; spf=pass (imf28.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1739386929; a=rsa-sha256; cv=none; b=dWzaCIg64H0tW9RApbFNx+FWkYu3w4s90nMUMQmmnjc/58N0v6AZ0JMAzdQxtinrffClUG la8I+tbRinWkdc0Ap2oCn+CH04WVQdNIyHuWsj5+FRl32loJ00SgqHKxSl3CLjVI4lm1l+ bBRf8PH7AAJ0nEDn9p+DT73HpQP4yoY= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=ZnibefHr; spf=pass (imf28.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1739386929; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=VnpKOCL9F8DbCzuG0WiiawmSIQd7cJlByrW6DA7MXYI=; b=sPsNM0H64XFMOB6Y+j/q0TOrbyHiQGGHFtSqpCgUWqWd1w/kDnDOugepNz9v3U0YOuUIVD FIYwFIeGgC8BnzC6iT5v+w96opHmhcKjTb+irA5FxT9WIuAYm5vV30Ec/UPVSrNYLKSY0Y owJi9ddAn/AXeDr/rKB48v++sEih7aM= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1739386921; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=VnpKOCL9F8DbCzuG0WiiawmSIQd7cJlByrW6DA7MXYI=; b=ZnibefHr7+TuoLb5Ok0oCXepYLE48mpFVf2xMZ/kuctZCnfNlpbZ7/5+QFwvF9KdSGaaQS JpZ8mToT4yYsdufWwwm/wCEoPAV5yNf9Nfq+hTjVai8rCnoLpbVF5Xv+qgHGQD9p06eom0 5tr5UQqYIRftT2mLgjpgsGIet8I3zGE= Received: from mail-wr1-f71.google.com (mail-wr1-f71.google.com [209.85.221.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-527-mX5AiNy8M8uQ7kSwW0Deog-1; Wed, 12 Feb 2025 14:01:41 -0500 X-MC-Unique: mX5AiNy8M8uQ7kSwW0Deog-1 X-Mimecast-MFC-AGG-ID: mX5AiNy8M8uQ7kSwW0Deog Received: by mail-wr1-f71.google.com with SMTP id ffacd0b85a97d-38dd8d11139so19114f8f.1 for ; Wed, 12 Feb 2025 11:01:38 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1739386891; x=1739991691; h=content-transfer-encoding:in-reply-to:organization:autocrypt :content-language:from:references:cc:to:subject:user-agent :mime-version:date:message-id:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=VnpKOCL9F8DbCzuG0WiiawmSIQd7cJlByrW6DA7MXYI=; b=EZju6cxetUUeAXvZteYZm6Z2NG31QI8aAWXYvIrfcH2uacPMF1CPdXagiFa0FgCTNx yLivJLfhqC7H8jetKRcemdWnioEBeukw5Rxwc74Lh/kbMGMbQtpjEL4sL8swkTvAjBsq 5m+5nMPkpSWZyKyYWgopRQldGRfT//d6QZeSEHvatCW/WSU8XgS9SDvfc1m/ZZSmcw1s pJ3PBVBVwlhUUS2WKw+lwAGMjfbRpC67V6K6082AYmnBT4/uSLqm3ZaRuIT+bhwad9Kn V9v5UudoSVpNbUIpRgvLIOW5qvTDLyFkmlqID17HZa0ceDpm3Bo3ehvgkB0vsQ87Rxo+ N1dQ== X-Forwarded-Encrypted: i=1; AJvYcCXfB0DQufOVnSPANVjKUPAOL27j9AiHcNy0LK9J9q/Q875BjHWcx3l9ZOq7xjBg39o3ymFmHecpew==@kvack.org X-Gm-Message-State: AOJu0YzZa1iBDSwCu2QsaX3MQnzitXfaFxK8mtXmNul24h2WdXN8DFjY 2vnd9DdGQ3f+74hRN2szr0yLW0wDEgka9G33fzMWlFTuz0++je/HXzO7HfkBw+1try3MqtLO6Hi VBrxxiPlcfJwyg7DuXSs1j6UnuaTwdi+LAvkm6u1OFR0TOmZ3 X-Gm-Gg: ASbGncv0oGb4MKYx37QX8wAs/anCOyB8hKiITj4JG8Sr34Bp2exN6KmXtFU8pUVVo/q e2R0xyOl9qSW1XvA2M1qr1Gp1Qx176iJqNGcq4Wn+XRyTPjfdEKxb/kDiqUwnHU836J/apYeDgk t9Alv/Gf2azBUvnx5v+dr4dZJdNIukhlHX0QRjRVEzKczU9X1oqEqHGD34qpSajsryWv6rrq2Wc r7OIH/uy8GP8OW6NOAhAGYGrDTbUm5QqjQELRga8AS9CUD7FI099xX/ojRJa64HOViIHIoutx8l Z/4nCKY/75kdichDdy6nuNKDgR2nG2TSws3xPTf3QYkyhpKQMd2WV60kHY0qH0HL4DpWFHhrCrN JKYln5T0E0J7RB0Sdy8FQnz0jxCPBPA== X-Received: by 2002:a05:6000:1fac:b0:38d:e584:81ea with SMTP id ffacd0b85a97d-38dea2f9ab6mr3331358f8f.45.1739386890862; Wed, 12 Feb 2025 11:01:30 -0800 (PST) X-Google-Smtp-Source: AGHT+IGMQEuHs2ARaifFarl1hlb7XuIU2Uf3rWR3SR85DWiFYlcoHcyJgYvPwxFa9XALu3tB9xPEVQ== X-Received: by 2002:a05:6000:1fac:b0:38d:e584:81ea with SMTP id ffacd0b85a97d-38dea2f9ab6mr3331310f8f.45.1739386890296; Wed, 12 Feb 2025 11:01:30 -0800 (PST) Received: from ?IPV6:2003:cb:c70c:a600:1e3e:c75:d269:867a? (p200300cbc70ca6001e3e0c75d269867a.dip0.t-ipconnect.de. [2003:cb:c70c:a600:1e3e:c75:d269:867a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38dc2f6aeafsm17010526f8f.20.2025.02.12.11.01.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 12 Feb 2025 11:01:29 -0800 (PST) Message-ID: Date: Wed, 12 Feb 2025 20:01:26 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/6] rust: page: Support borrowing `struct page` and physaddr conversion To: Asahi Lina , Zi Yan Cc: Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Jann Horn , Matthew Wilcox , Paolo Bonzini , Danilo Krummrich , Wedson Almeida Filho , Valentin Obst , Andrew Morton , linux-mm@kvack.org, airlied@redhat.com, Abdiel Janulgue , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, asahi@lists.linux.dev, Oscar Salvador , Muchun Song References: <20250202-rust-page-v1-0-e3170d7fe55e@asahilina.net> <41ca3445-80cd-43c1-8f9e-634c195c9187@asahilina.net> <37A0729B-A711-4D45-B9F0-328FDB9ADD28@nvidia.com> <0e19e1c3-293b-4740-93f3-2c410893288b@redhat.com> <82047858-480a-45e3-b826-3a46fbebe842@asahilina.net> <1e9ae833-4293-4e48-83b2-c0af36cb3fdc@asahilina.net> <026c1a0c-e53a-4a5e-92da-6e4f18ce0fee@redhat.com> <6bcd3315-a0f9-463c-ab97-a43736f9b4f4@redhat.com> <2a513c3e-818c-4040-b3d3-7835861bab4f@asahilina.net> <0dffaa7d-340f-4ce1-9a2e-54cfd9079266@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: nBl3K88YZhhvP5_2mEQPKWhlXzpxWSBqgu58EZr6L6k_1739386891 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 708AAC001B X-Stat-Signature: f1c41rmgd9wbrjirwwfzc5crm64jxhnd X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1739386926-573406 X-HE-Meta: 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 k09OPdc4 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.435649, 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 06.02.25 20:18, Asahi Lina wrote: > > > On 2/7/25 2:58 AM, David Hildenbrand wrote: >> On 04.02.25 22:06, Asahi Lina wrote: >>> >>> >>> On 2/5/25 5:10 AM, David Hildenbrand wrote: >>>> On 04.02.25 18:59, Asahi Lina wrote: >>>>> On 2/4/25 11:38 PM, David Hildenbrand wrote: >>>>>>>>> If the answer is "no" then that's fine. It's still an unsafe >>>>>>>>> function >>>>>>>>> and we need to document in the safety section that it should >>>>>>>>> only be >>>>>>>>> used for memory that is either known to be allocated and pinned and >>>>>>>>> will >>>>>>>>> not be freed while the `struct page` is borrowed, or memory that is >>>>>>>>> reserved and not owned by the buddy allocator, so in practice >>>>>>>>> correct >>>>>>>>> use would not be racy with memory hot-remove anyway. >>>>>>>>> >>>>>>>>> This is already the case for the drm/asahi use case, where the pfns >>>>>>>>> looked up will only ever be one of: >>>>>>>>> >>>>>>>>> - GEM objects that are mapped to the GPU and whose physical >>>>>>>>> pages are >>>>>>>>> therefore pinned (and the VM is locked while this happens so the >>>>>>>>> objects >>>>>>>>> cannot become unpinned out from under the running code), >>>>>>>> >>>>>>>> How exactly are these pages pinned/obtained? >>>>>>> >>>>>>> Under the hood it's shmem. For pinning, it winds up at >>>>>>> `drm_gem_get_pages()`, which I think does a >>>>>>> `shmem_read_folio_gfp()` on >>>>>>> a mapping set as unevictable. >>>>>> >>>>>> Thanks. So we grab another folio reference via shmem_read_folio_gfp()- >>>>>>> shmem_get_folio_gfp(). >>>>>> >>>>>> Hm, I wonder if we might end up holding folios residing in >>>>>> ZONE_MOVABLE/ >>>>>> MIGRATE_CMA longer than we should. >>>>>> >>>>>> Compared to memfd_pin_folios(), which simulates FOLL_LONGTERM and >>>>>> makes >>>>>> sure to migrate pages out of ZONE_MOVABLE/MIGRATE_CMA. >>>>>> >>>>>> But that's a different discussion, just pointing it out, maybe I'm >>>>>> missing something :) >>>>> >>>>> I think this is a little over my head. Though I only just realized that >>>>> we seem to be keeping the GEM objects pinned forever, even after unmap, >>>>> in the drm-shmem core API (I see no drm-shmem entry point that would >>>>> allow the sgt to be freed and its corresponding pages ref to be >>>>> dropped, >>>>> other than a purge of purgeable objects or final destruction of the >>>>> object). I'll poke around since this feels wrong, I thought we were >>>>> supposed to be able to have shrinker support for swapping out whole GPU >>>>> VMs in the modern GPU MM model, but I guess there's no >>>>> implementation of >>>>> that for gem-shmem drivers yet...? >>>> >>>> I recall that shrinker as well, ... or at least a discussion around it. >>>> >>>> [...] >>>> >>>>>> >>>>>> If it's only for crash dumps etc. that might even be opt-in, it makes >>>>>> the whole thing a lot less scary. Maybe this could be opt-in >>>>>> somewhere, >>>>>> to "unlock" this interface? Just an idea. >>>>> >>>>> Just to make sure we're on the same page, I don't think there's >>>>> anything >>>>> to unlock in the Rust abstraction side (this series). At the end of the >>>>> day, if nothing else, the unchecked interface (which the regular >>>>> non-crash page table management code uses for performance) will let you >>>>> use any pfn you want, it's up to documentation and human review to >>>>> specify how it should be used by drivers. What Rust gives us here is >>>>> the >>>>> mandatory `unsafe {}`, so any attempts to use this API will necessarily >>>>> stick out during review as potentially dangerous code that needs extra >>>>> scrutiny. >>>>> >>>>> For the client driver itself, I could gate the devcoredump stuff behind >>>>> a module parameter or something... but I don't think it's really worth >>>>> it. We don't have a way to reboot the firmware or recover from this >>>>> condition (platform limitations), so end users are stuck rebooting to >>>>> get back a usable machine anyway. If something goes wrong in the >>>>> crashdump code and the machine oopses or locks up worse... it doesn't >>>>> really make much of a difference for normal end users. I don't think >>>>> this will ever really happen given the constraints I described, but if >>>>> somehow it does (some other bug somewhere?), well... the machine was >>>>> already in an unrecoverable state anyway. >>>>> >>>>> It would be nice to have userspace tooling deployed by default that >>>>> saves off the devcoredump somewhere, so we can have a chance at >>>>> debugging hard-to-hit firmware crashes... if it's opt-in, it would only >>>>> really be useful for developers and CI machines. >>>> >>>> Is this something that possibly kdump can save or analyze? Because that >>>> is our default "oops, kernel crashed, let's dump the old content so we >>>> can dump it" mechanism on production systems. >>> >>> kdump does not work on Apple ARM systems because kexec is broken and >>> cannot be fully fixed, due to multiple platform/firmware limitations. A >>> very limited version of kexec might work well enough for kdump, but I >>> don't think anyone has looked into making that work yet... >>> >>>> but ... I am not familiar with devcoredump. So I don't know when/how it >>>> runs, and if the source system is still alive (and remains alive --  in >>>> contrast to a kernel crash). >>> >>> Devcoredump just makes the dump available via /sys so it can be >>> collected by the user. The system is still alive, the GPU is just dead >>> and all future GPU job submissions fail. You can still SSH in or (at >>> least in theory, if enough moving parts are graceful about it) VT-switch >>> to a TTY. The display controller is not part of the GPU, it is separate >>> hardware. >> >> >> Thanks for all the details (and sorry for the delay, I'm on PTO until >> Monday ... :) >> >> (regarding the other mail) Adding that stuff to rust just so we have a >> devcoredump that ideally wouldn't exist is a bit unfortunate. >> >> So I'm curious: we do have /proc/kcore, where we do all of the required >> filtering, only allowing for reading memory that is online, not >> hwpoisoned etc. >> >> makedumpfile already supports /proc/kcore. >> >> Would it be possible to avoid Devcoredump completely either by dumping / >> proc/kcore directly or by having a user-space script that walks the page >> tables to dump the content purely based on /proc/kcore? >> >> If relevant memory ranges are inaccessible from /proc/kcore, we could >> look into exposing them. > > I'm not sure that's a good idea... the dump code runs when the GPU > crashes, and makes copies of all the memory pages into newly allocated > pages (this is around 16MB for a typical dump, and if allocation fails > we just bail and clean up). Then userspace can read the coredump at its > leisure. AIUI, this is exactly the intended use case of devcoredump. It > also means that anyone can grab a core dump with just a `cp`, without > needing any bespoke tools. > > After the snapshot is taken, the kernel will complete (fail) all GPU > jobs, which means much of the shared memory will be freed and some > structures will change contents. Ah, okay, that's an issue. > If we defer the coredump to userspace, > then it would not be able to capture the state of all relevant memory > exactly at the crash time, which could be very confusing. > > In theory I could change the allocators to not free or touch anything > after a crash, and add guards to any mutations in the driver to avoid > any changes after a crash... but that feels a lot more brittle and > error-prone than just taking the core dump at the right time. Agreed. -- Cheers, David / dhildenb