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 D0674440A03; Tue, 28 Jul 2026 13:41:06 +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=1785246068; cv=none; b=ZTQPFOLlXmwJEyMrYktNpDT7ImAx85vzfqWY0uSVzkb0DQjl2plU+2WO4gbXDN08grJWGjMKr6+BzFchSf7bVSX1rx52kVj+Z0IWNUN7lM+skaYKScRldArOpFMnZeW0ilbzF51am8MmnTCx3bnY/JOMit5I0z0f0fXVwcz9TZ0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785246068; c=relaxed/simple; bh=d519tbNUY1zX6k442JW0TgBR7NVbPaiBhL5Lk+UuOWA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nZuAe9Nc5wjH4GayrZc6m355TY8DMWcD43+gOGxiXFnyZp3AAp4EDutJRpWgFC7+bkLMeFwed+GXjDcZ3Q241Gmqxg7zrUBEB6wFtPvUHWBCrEnkWeUp+dgo1A3YF016puh830n5HZy8qH7mgYDodyaCmcBR3a/4HHvDiBMOkvg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=U6rCQrh5; 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="U6rCQrh5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A50981F000E9; Tue, 28 Jul 2026 13:40:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785246066; bh=HEMU2TmSwsqrUTy8ZEKYu9Ae0PHGeT+f/ud0LSQJwt8=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=U6rCQrh5cCM83BFeZR9/Qvz6ZbhtwuYWaehIe+rVPnqsrpeoZxZ7B10TBBl8W6i2v bNl8xdz3N7EGPECsNHEUwcq7V6eXM065C3EsgL84YXJLnqzq6/63+lR4ajgwFmWjia JkPs+1kKRodOHGdKHGtsSMAqqnX55EzHnTzrwhr6R+NPj/XCwR1rYWbkoP2dlqZOty fHKBsYlxdrZfaBsgyDoTQfTbzFICpH6JzdT1q8XsyCZQSIbVk30IicsnJBR+9DqOCg GcNUnIcJVMkBfM1LBlYvBh6QQsY2EOWD4OvET/dMUu0/DjKekDYnPntoQpf0AZbORa tMd75YGtaicKw== Date: Tue, 28 Jul 2026 14:40:44 +0100 From: "Lorenzo Stoakes (ARM)" To: Matthew Wilcox Cc: "David Hildenbrand (Arm)" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , 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: On Mon, Jul 27, 2026 at 04:59:54PM +0100, Matthew Wilcox wrote: > On Mon, Jul 27, 2026 at 04:57:01PM +0100, Lorenzo Stoakes (ARM) wrote: > > 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. > > Would the term "logical page offset" be helpful? > > I already use this in my description of folios: > > * A folio is a physically, virtually and logically contiguous set > * of bytes. It is a power-of-two in size, and it is aligned to that > * same power-of-two. It is at least as large as %PAGE_SIZE. If it is > * in the page cache, it is at a file offset which is a multiple of that > * power-of-two. It may be mapped into userspace at an address which is > * at an arbitrary page offset, but its kernel virtual address is aligned > * to its size. > Thanks, I was kinda getting at this sort of thing, and I did think 'virtual page offset' was a fairly common term of art, but as often with these things you go and look to check later and realise that maybe it's not :P I think overall it's probably easier to go to anon everywhere, I'll eyeball it when done and make sure it's sane though! Cheers, Lorenzo