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 B0D003FB067; Mon, 27 Jul 2026 15:57:22 +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=1785167843; cv=none; b=SwUpCZuTBY5kxmJzaaxUP9RnGEfyTPWcod0cPC/ixWW9jC1HoRqeQgfMa64qFXqCcDRSR2OibcJcwWUPCjVG2HXqdoQ7YJ1DB7F+q23opr/hh/xMW6Lq+8HnO3uXgNUMIoSJrRGA4qOC3/mxHpCN5DfG9mZpMUmq2vxeQvMkVyM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785167843; c=relaxed/simple; bh=lrs1wWLpaJK6eWPjL/KCSd3E0ddkjJ1+NsCq+5dynfM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kE9/r9OjoqvhSH86zAaTNiiS8Bqb6kyPOQYdSDQ7CUkrQ6zRQXle27qoS714j84R4HrrU3Mi7ijmRg6mF3v9ZSLRgNLv8RyRoO0ZXUbad2DXhQqC1/FFEDH3bDrC9tF3+5utxZKt0IF1iAvrOkp1SLCUZfjzK3RTuNlv7nnr7jo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=aS3LgLoe; 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="aS3LgLoe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 610571F00A3A; Mon, 27 Jul 2026 15:57:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785167842; bh=TxIwg3k/QeB5YpM8mpnKKHWqe4ae+QJvkXRnO46mmqU=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=aS3LgLoemsT7sttTQz1bFcjtnuSJmD82qShxfwVkKEIVhwyCRQzFYrZ1BGt+oX2Ad Zf/cPw46qAZ1uENRq/ddMbAK89LecqDJ3Nd4sMeOUAJ/n9cCYEr7gF8uvY/wmGkqEi PmuSEGwiQlipw0ThXrzYwa+jDqQoj+bcbOCcTcwcmHibSGq1zid3nrfThmk2upJTjh G1Hetwq6wII0F3cwpJrvE1YwRvjRhA/4Gg8q9NTwPKYsXDBqFCTCLb2FSEAN96lqE1 rq3yR6XbderfGQLXvgkBaS7Z51i7zc6vDGwxsSYeXh2GlmRQefluWQuJ1X+Kq2YG9U wmDqydDlG77aA== Date: Mon, 27 Jul 2026 16:57:01 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: 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 , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v2 00/15] mm/rmap: index MAP_PRIVATE file-backed folios by virt pgoff Message-ID: References: <20260720-b4-scalable-cow-virt-pgoff-v2-0-2d549757a76f@kernel.org> <880981b5-ef79-4c9a-a8bd-7540002fcd15@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <880981b5-ef79-4c9a-a8bd-7540002fcd15@kernel.org> On Mon, Jul 27, 2026 at 04:44:06PM +0200, David Hildenbrand (Arm) wrote: > 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. Yeah, I had originally done this. I renamed it because I worried that people might be confused by references to anon for file-backed VMA mappings. But I think probably you're right that referring to it as virtual page offset adds even more confusion vs. simply calling it anon. Will respin accordingly. > > 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); > } Ack yeah. > > > 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. No it was just fear of adding more confusion but seems by doing so I've done the reverse of what I wanted :P When I rework it if I notice anything that promotes this as a better solution will ping otherwise will just rename on respin. > > -- > Cheers, > > David Cheers, Lorenzo