From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 08C6C23185E for ; Fri, 25 Jul 2025 09:52:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753437123; cv=none; b=HoJomWOZgUROySNexa3Jyl3ZOJitCehcc4JxeiLK9hqVKYOl4qWjraTDqHPIpyf3OOwqNNsY72MPuP4Jab6iybPToyY4aiVHnMMUM9xFT8QSf77zjeVl56MYzltMOtZmwRz2mnAEahCUmh5s4mVEpb7WwfQwhljOIE7t7Mmm7mA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753437123; c=relaxed/simple; bh=LUwUHsA5yyl9/cHcI5QJeAZ2v8M1Sdik9K/naFteEPE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QOea1j6ufQr+PGNBNu0iPLPgLnctFekGv7Zix27X+xedSqzPNKaS2yjc+mPW8DFggjyQ6Eitx31ddvKDKUlsKcCBOFZLEuSycTrE4vN9CPl3DQsKkHme+z470Br9lTorafZAE7rvebwwVa3b+2BvF0WhPHgeRZYi8tLNipMh00M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=VtbgN8ON; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="VtbgN8ON" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1753437120; 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=MTWdy+1SlxCZIWCULVABxeLHJl6jZUNfpstFC/dkyck=; b=VtbgN8ON50nAd5lpCsB5eU/ur5Ud1CZDeHZZTRq9qiFFCjYA67eF7BSB73HmD/XgM3UdSK qVyQYFSd7RgM7aF9ETOac4xaTs7RGHBnFDEhORbQGLhRrZl0DOpttWnFLyoWl9FER+pbl5 MyYCQg+Qe8PV3S9xwtAeqF3zmMnXhJk= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-199-BY7NuOcxOOm9Of-N-FEUTw-1; Fri, 25 Jul 2025 05:51:58 -0400 X-MC-Unique: BY7NuOcxOOm9Of-N-FEUTw-1 X-Mimecast-MFC-AGG-ID: BY7NuOcxOOm9Of-N-FEUTw_1753437117 Received: by mail-wm1-f71.google.com with SMTP id 5b1f17b1804b1-4586cc8f9f2so4203775e9.2 for ; Fri, 25 Jul 2025 02:51:58 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1753437117; x=1754041917; 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=MTWdy+1SlxCZIWCULVABxeLHJl6jZUNfpstFC/dkyck=; b=f1J0cGmYR1mBLKkSvUdlif4ysPvgZrjBCMN5zj0TWBrV01wtZzr27wJeN/zg/qV8uL K2U1EYJOG9jDRQTSIJcvxv4gdKeTIhoMdVEm/2uDSbVrWJexUTzNJc+9g2Euxuxnpoul fKazhEuX9kKC8Gg8EzufMbQHZNDNmzy83Xjm5ciHlWFh9JoJN/RTBVLCSD+Bb3VQkg/8 aVSbxhT84dWuzjkCU7TtJZEWp+9yV4PYncVt1uZ+WlGcJeYKYRXLtn0R4L+jtJOV9dgn dU7QS1Koewwn1XclrEyGRNJoTHhsr6tDhYVMv5d3WJyqVTH57tkQ844MkwOV2MU/rs9q MLPw== X-Forwarded-Encrypted: i=1; AJvYcCVlpUa6yO4QqD6iq850yP0UiCT+BPxAO1ERWUHJoKHmxwObj4oogcxTPBaO8LnJql5lqXMa8TOFjiuu@vger.kernel.org X-Gm-Message-State: AOJu0YxtLudpoDH7y+qMTpLoBzCsxSJ8+ARtaRxcnpanfHdORUjRuoA1 fatiarPzDxzYCBuuv/kb4Xk7n32UIXOefg4QHeNB/a0ufgcn8E976Ku+ixRGRsHmUva51WuohHM qu52lGOdhvlp0/AlSKk3RHRfHMORfLjx3vPFTkKttAkB53A1qBnyYzcx91soXf8g= X-Gm-Gg: ASbGncvqxdJF1x6KqcIaV8egYPocbi5Rfj/lbcdD4F8WODVBIjuwHXoAB3od9TqULu8 15wCFpH51kfxuesiBct4ghJ6VYvoxVW9L8mFfEy2XRd+yxEBuefuE4MU2RB+E0j7D5AsAyMOdfx yGYc8yqu9XrmXHdkMpM2mxD+Hj7b4OS+niAl/YdMqEus/JAAMgQhAEqYWRaoBbarlKdJxdpVBqj jf78tQRbQEMg3VuCpZNqoik1OMbT3TGqzjtAjMhuRzLPS/O90zHObymHaQk6XEYBKBAX62bn68K 2wB+E7wWrzL2ZAdYRaNWepOeQ9GDlH3lV3Wgd8y8xnfGjIDC2QOW6UZZZfDmhR74aBpuGhKBcIU 0eyNWYvtmgj1cLcQ4iZY7DymXak6XqyuEAklKCEIDUBtC8Lf6agpdEOsd5/cyuRoOBk8= X-Received: by 2002:a05:6000:420c:b0:3a6:d296:feaf with SMTP id ffacd0b85a97d-3b776737031mr1174849f8f.24.1753437117259; Fri, 25 Jul 2025 02:51:57 -0700 (PDT) X-Google-Smtp-Source: AGHT+IF2Ju9qbPhWwNNwVGPWubTU0EaExWPb4CE4F4jST4yWvzRfvTD7NkOT+SB5u1CFnRPEUIy3bw== X-Received: by 2002:a05:6000:420c:b0:3a6:d296:feaf with SMTP id ffacd0b85a97d-3b776737031mr1174815f8f.24.1753437116691; Fri, 25 Jul 2025 02:51:56 -0700 (PDT) Received: from ?IPV6:2003:d8:2f1a:f400:5a9f:b1bf:4bb3:99b1? (p200300d82f1af4005a9fb1bf4bb399b1.dip0.t-ipconnect.de. [2003:d8:2f1a:f400:5a9f:b1bf:4bb3:99b1]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b7785d5e31sm207596f8f.6.2025.07.25.02.51.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 25 Jul 2025 02:51:56 -0700 (PDT) Message-ID: Date: Fri, 25 Jul 2025 11:51:54 +0200 Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device private pages To: Alistair Popple Cc: Jason Gunthorpe , Matthew Wilcox , Yonatan Maman , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Andrew Morton , Leon Romanovsky , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , Ben Skeggs , Michael Guralnik , Or Har-Toov , Daisuke Matsuda , Shay Drory , linux-mm@kvack.org, linux-rdma@vger.kernel.org, dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org, linux-kernel@vger.kernel.org, Gal Shalom References: <20250718115112.3881129-1-ymaman@nvidia.com> <20250718115112.3881129-2-ymaman@nvidia.com> <20250718144442.GG2206214@ziepe.ca> <7lvduvov3rvfsgixbkyyinnzz3plpp3szxam46ccgjmh6v5d7q@zoz4k723vs3d> 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/g1oFAmgsLPQFCRvGjuMACgkQTd4Q 9wD/g1o0bxAAqYC7gTyGj5rZwvy1VesF6YoQncH0yI79lvXUYOX+Nngko4v4dTlOQvrd/vhb 02e9FtpA1CxgwdgIPFKIuXvdSyXAp0xXuIuRPQYbgNriQFkaBlHe9mSf8O09J3SCVa/5ezKM OLW/OONSV/Fr2VI1wxAYj3/Rb+U6rpzqIQ3Uh/5Rjmla6pTl7Z9/o1zKlVOX1SxVGSrlXhqt kwdbjdj/csSzoAbUF/duDuhyEl11/xStm/lBMzVuf3ZhV5SSgLAflLBo4l6mR5RolpPv5wad GpYS/hm7HsmEA0PBAPNb5DvZQ7vNaX23FlgylSXyv72UVsObHsu6pT4sfoxvJ5nJxvzGi69U s1uryvlAfS6E+D5ULrV35taTwSpcBAh0/RqRbV0mTc57vvAoXofBDcs3Z30IReFS34QSpjvl Hxbe7itHGuuhEVM1qmq2U72ezOQ7MzADbwCtn+yGeISQqeFn9QMAZVAkXsc9Wp0SW/WQKb76 FkSRalBZcc2vXM0VqhFVzTb6iNqYXqVKyuPKwhBunhTt6XnIfhpRgqveCPNIasSX05VQR6/a OBHZX3seTikp7A1z9iZIsdtJxB88dGkpeMj6qJ5RLzUsPUVPodEcz1B5aTEbYK6428H8MeLq NFPwmknOlDzQNC6RND8Ez7YEhzqvw7263MojcmmPcLelYbfOwU0EVcufkQEQAOfX3n0g0fZz 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+DWgUCaCwtJQUJG8aPFAAKCRBN3hD3AP+DWlDnD/4k2TW+HyOOOePVm23F5HOhNNd7nNv3 Vq2cLcW1DteHUdxMO0X+zqrKDHI5hgnE/E2QH9jyV8mB8l/ndElobciaJcbl1cM43vVzPIWn 01vW62oxUNtEvzLLxGLPTrnMxWdZgxr7ACCWKUnMGE2E8eca0cT2pnIJoQRz242xqe/nYxBB /BAK+dsxHIfcQzl88G83oaO7vb7s/cWMYRKOg+WIgp0MJ8DO2IU5JmUtyJB+V3YzzM4cMic3 bNn8nHjTWw/9+QQ5vg3TXHZ5XMu9mtfw2La3bHJ6AybL0DvEkdGxk6YHqJVEukciLMWDWqQQ RtbBhqcprgUxipNvdn9KwNpGciM+hNtM9kf9gt0fjv79l/FiSw6KbCPX9b636GzgNy0Ev2UV m00EtcpRXXMlEpbP4V947ufWVK2Mz7RFUfU4+ETDd1scMQDHzrXItryHLZWhopPI4Z+ps0rB CQHfSpl+wG4XbJJu1D8/Ww3FsO42TMFrNr2/cmqwuUZ0a0uxrpkNYrsGjkEu7a+9MheyTzcm vyU2knz5/stkTN2LKz5REqOe24oRnypjpAfaoxRYXs+F8wml519InWlwCra49IUSxD1hXPxO WBe5lqcozu9LpNDH/brVSzHCSb7vjNGvvSVESDuoiHK8gNlf0v+epy5WYd7CGAgODPvDShGN g3eXuA== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 25.07.25 02:31, Alistair Popple wrote: > On Thu, Jul 24, 2025 at 10:52:54AM +0200, David Hildenbrand wrote: >> On 23.07.25 06:10, Alistair Popple wrote: >>> On Wed, Jul 23, 2025 at 12:51:42AM -0300, Jason Gunthorpe wrote: >>>> On Tue, Jul 22, 2025 at 10:49:10AM +1000, Alistair Popple wrote: >>>>>> So what is it? >>>>> >>>>> IMHO a hack, because obviously we shouldn't require real physical addresses for >>>>> something the CPU can't actually address anyway and this causes real >>>>> problems >>>> >>>> IMHO what DEVICE PRIVATE really boils down to is a way to have swap >>>> entries that point to some kind of opaque driver managed memory. >>>> >>>> We have alot of assumptions all over about pfn/phys to page >>>> relationships so anything that has a struct page also has to come with >>>> a fake PFN today.. >>> >>> Hmm ... maybe. To get that PFN though we have to come from either a special >>> swap entry which we already have special cases for, or a struct page (which is >>> a device private page) which we mostly have to handle specially anyway. I'm not >>> sure there's too many places that can sensibly handle a fake PFN without somehow >>> already knowing it is device-private PFN. >>> >>>>> (eg. it doesn't actually work on anything other than x86_64). There's no reason >>>>> the "PFN" we store in device-private entries couldn't instead just be an index >>>>> into some data structure holding pointers to the struct pages. So instead of >>>>> using pfn_to_page()/page_to_pfn() we would use device_private_index_to_page() >>>>> and page_to_device_private_index(). >>>> >>>> It could work, but any of the pfn conversions would have to be tracked >>>> down.. Could be troublesome. >>> >>> I looked at this a while back and I'm reasonably optimistic that this is doable >>> because we already have to treat these specially everywhere anyway. >> How would that look like? >> >> E.g., we have code like >> >> if (is_device_private_entry(entry)) { >> page = pfn_swap_entry_to_page(entry); >> folio = page_folio(page); >> >> ... >> folio_get(folio); >> ... >> } >> >> We could easily stop allowing pfn_swap_entry_to_page(), turning these into >> non-pfn swap entries. >> >> Would it then be something like >> >> if (is_device_private_entry(entry)) { >> page = device_private_entry_to_page(entry); >> >> ... >> } >> >> Whereby device_private_entry_to_page() obtains the "struct page" not via the >> PFN but some other magical (index) value? > > Exactly. The observation being that when you convert a PTE from a swap entry > to a page we already know it is a device private entry, so can go look up the > struct page with special magic (eg. an index into some other array or data > structure). > > And if you have a struct page you already know it's a device private page so if > you need to create the swap entry you can look up the magic index using some > alternate function. > > The only issue would be if there were generic code paths that somehow have a > raw pfn obtained from neither a page-table walk or struct page. My assumption > (yet to be proven/tested) is that these paths don't exist. I guess memory compaction and friends don't apply to ZONE_DEVICE, and even memory_failure() handling goes a separate path. -- Cheers, David / dhildenb