From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 81B3C415F16; Mon, 27 Jul 2026 14:44:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785163468; cv=none; b=CbeIgt0RDfczQZBalSpLJvkTfXwLw3+/8AzccuAiMEKVnwWE7TL/cIBI9z53Pk5TeqJXzO8k+QEJwODjMAY14jucpv7rtsg49XWutTDna8fRHoSzrLl3oxfmkJKmIZNuSDOrM8J5+iGqrICKcuXhsCYG8uOTMLaG8U4b7DhfrTg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785163468; c=relaxed/simple; bh=FcNe3Ar+PqQPRGDU/m8/ewnjWtyFw8x7wrTIstDIwHE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=NQUF35MuXplZN4ZHbnyuAoSobsn+zGEcSaRhTFYJOW1p9zuhKh/P0R1KHINxmTVhK18uuebe3WhXOXYBfgMqU55U/KNdzQLCDcrKc4KFkeybOecx4ajuOQdYitcNqSGyp2Nj4qlhdUgRzcpypSGWcKGu2T8di6ztvUFRCbbIN+Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PhVGoeHy; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PhVGoeHy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8CFF11F00A3E; Mon, 27 Jul 2026 14:44:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785163459; bh=9dVdzXw/O9Qo+fRyZpEM/TgP7hh11wbOH6xloCAG0ak=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PhVGoeHy8JxQDq4dBfU7UzadtQTNbux9yx1M1nF5IWCDPKsj6+e+vVpNWfQ3LmbNE tO2Oqj2Y3nB16V5VyHdG1HtWXKtboMn3dFVzmNjfoukR3RA0HlM8qBYHKrhA2kCD6R NKWMUdkc3IKrdL3lqS+seJQmoAEIW5L3BqGnjoyYCM0ycRaCZNBVeKYt+ankm19Efw kK/ekAkBII7oXh30iRWfV2WGvHhBMZulq8v8Tgo892r5+67o2AhLWkjx/BW9eVFDgd Aa9/jGsEDb6g34eDMzUY0qkDMsaUzTTOhuFJ/VNifyyud1Oh6HIiQ1ETvBt6/3zBZl jtSwRrAVu8J7A== Message-ID: <880981b5-ef79-4c9a-a8bd-7540002fcd15@kernel.org> Date: Mon, 27 Jul 2026 16:44:06 +0200 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 v2 00/15] mm/rmap: index MAP_PRIVATE file-backed folios by virt pgoff To: "Lorenzo Stoakes (ARM)" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , "Matthew Wilcox (Oracle)" , Jan Kara , Miaohe Lin , Naoya Horiguchi , Rik van Riel , Harry Yoo , Lance Yang , Kees Cook , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Usama Arif , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Xu Xin , Chengming Zhou , Arnd Bergmann , Greg Kroah-Hartman Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260720-b4-scalable-cow-virt-pgoff-v2-0-2d549757a76f@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260720-b4-scalable-cow-virt-pgoff-v2-0-2d549757a76f@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/20/26 16:38, Lorenzo Stoakes (ARM) wrote: > In memory management we've managed to manufacture a great deal of confusion > around the concept of anonymous memory. We have: > > 1. 'Pure anon' memory - anonymous VMAs whose folios are anonymous and > swap-backed (thus for reclaim purposes, treated as anonymous). These are > simple enough. > > 2. shmem - file-backed VMAs, file-backed folios (from rmap perspective) so > present in the page cache and mapped by an address_space object, but > whose folios are also swap-backed (thus treated as anonymous for reclaim > purposes). > > 3. MAP_PRIVATE-mapped /dev/zero - a strange beast whose VMAs have > vma->vm_file set, but whose mmap_prepare callback clears vma->vm_ops to > satisfy vma_is_anonymous(), which results in VMAs that were mmap()'d > referencing a file, but are in every other sense anonymous, including the > folios. > > 4. Other MAP_PRIVATE-file backed mappings - These possess file-backed VMAs > and have file-backed folios until CoW'd, at which point those CoW'd > folios are anonymous. > > This series fixes issues 3 and 4. > > In order for us to traverse VMAs using the reverse mapping, we require two > fields - folio->mapping and folio->index. The first tells the rmap code > where to look for VMAs, and the second tells it at which offset the folio > starts within the referenced object. > > For anonymous folios, folio->mapping points at an anon_vma object. For > file-backed folios, it points at an address_space. And: > > * For file-backed folios folio->index is simply the page offset of the start > of the folio within the file. > > * For anonymous folios belonging to pure anon mappings, folio->index is > equal to the virtual page offset of the folio. > > * For anonymous folios belonging to file-backed mappings (i.e. CoW'd folios > of a MAP_PRIVATE file-backed mapping), folio->index is equal to the file > page offset. > > This series establishes a new virtual page offset property of VMAs to > allow us to map anonymous folios at their virtual page offset, consistent > with pure anon. As raised off-list, I consider the "virtual page offset" concept hard to grasp. Maybe it's just me :) Skimming over the code, I read "vmg->anon_pgoff", which is pretty intuitive to me. Similarly vma_start_anon_pgoff() / vmg_end_anon_pgoff(). Could we similarly just call this "anon_pgoff" / "(linear) anon page index" even on the VMA level. IOW, in patch #9 for example: static inline pgoff_t linear_folio_page_index(const struct folio *folio, const struct vm_area_struct *vma, const unsigned long address) { if (folio_test_anon(folio)) + return linear_anon_page_index(vma, address); + return linear_page_index(vma, address); } Or is there another user for the the "virtual page offset" concept? I'd assume it's only used for anon folios (including KSM), but maybe I am missing some corner case. -- Cheers, David