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 AB07C3314AE; Mon, 10 Aug 2026 22:10:10 +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=1786399811; cv=none; b=hlSWU4gbsH9UY6A+r4gZEjgdVHnKwIupQPCa5Xz1RlDNTelu8fNA0OUlNJPLr4oSTci8wsdYTmKE3KipMuzzevgnjaaiVssYZz6NE8s607CQvuFMaBn3iUmDcYkJlDz8KO/RZSa9AfjKwjim2mQWi0KKzULYPhCWGOah0oXPKyY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786399811; c=relaxed/simple; bh=0RWpdfIzCKEiyFzszpDc1xkHvtl02bntYX24+KDlego=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=qj8jAECR6HfvLNZWPvRCGW+dv4oD+gMp3zR10Rx3dJ56Ap7Z20lAySDZXQXBEkEQjGkCE9k227c/T7ZCccvVleVy0WgkdwYU7o2cuTY9BVr/IywsO6fOyngywA8nj+tfHR2qKesA8aw3EySDSU75BrPdlm45qHmGneyxG0GT3Ew= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=UwX/Ilov; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="UwX/Ilov" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AB66F1F000E9; Mon, 10 Aug 2026 22:10:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1786399810; bh=/hUK9nmdYQUfhG6ut1/QpR/T7D3LLI4jSc7+bBZ5Ofc=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=UwX/Ilov0bIFii9m3iwtL8q7dWNlzT+Wx3UroPo9PylX7if0NcfH9JAavRDAn86dw JNwDAbfcVY3VRo/UBsUbAdQe6WqOiVdxquE1mb7LVSCrY0NRcx31i22SVoR5H3wZ9K Nroa/JyRqqFqLrw3hFrR5C55A7jEiwD4QPiMGP18= Date: Mon, 10 Aug 2026 15:10:07 -0700 From: Andrew Morton To: "Lorenzo Stoakes (ARM)" Cc: David Hildenbrand , "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 , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Sven Schnelle , Alex Deucher , Christian =?ISO-8859-1?Q?K=F6nig?= , David Airlie , Simona Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Boris Brezillon , Steven Price , Liviu Dudau , Huang Rui , Matthew Auld , Thomas =?ISO-8859-1?Q?Hellstr=F6m?= , Rodrigo Vivi , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , Jason Gunthorpe , John Hubbard , Muchun Song , Oscar Salvador , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org, linux-s390@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-perf-users@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH v4 02/20] mm: provide vma_[flags_]is_cow_mapping() and remove is_cow_mapping() Message-Id: <20260810151007.c1cfbcf46d498d2f8ce566f9@linux-foundation.org> In-Reply-To: References: <20260806-b4-scalable-cow-virt-pgoff-v4-0-ab318a350404@kernel.org> <20260806-b4-scalable-cow-virt-pgoff-v4-2-ab318a350404@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 10 Aug 2026 18:59:34 +0100 "Lorenzo Stoakes (ARM)" wrote: > Hi Andrew - would it be possible to insert the comments added below? Trying > to avoid a respin if possible :) np, like this? From: Andrew Morton Subject: mm-provide-vma_is_cow_mapping-and-remove-is_cow_mapping-fix Date: Mon Aug 10 03:07:02 PM PDT 2026 add comments, per Lorenzo Link: https://lore.kernel.org/ann-vS6MYT9kuXqu@lucifer Cc: "Lorenzo Stoakes (ARM)" Signed-off-by: Andrew Morton --- include/linux/mm.h | 57 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) --- a/include/linux/mm.h~mm-provide-vma_is_cow_mapping-and-remove-is_cow_mapping-fix +++ a/include/linux/mm.h @@ -2271,17 +2271,74 @@ void unpin_user_pages(struct page **page void unpin_user_folio(struct folio *folio, unsigned long npages); void unpin_folios(struct folio **folios, unsigned long nfolios); +/** + * vma_flags_is_cow_mapping() - Do these VMA flags imply a CoW mapping? + * @flags: The VMA flags to check. + * + * Mappings which could be CoW'd (subject to Copy-On-Write faults) are + * described as CoW mappings. + * + * All mappings backed by anonymous folios (all anonymous mappings and most + * MAP_PRIVATE-file backed ranges) are CoW mappings. + * + * All other mappings (including all writable MAP_SHARED mappings) are + * non-CoW. + * + * The criteria are !VMA_SHARED_BIT, VMA_MAYWRITE_BIT. + * + * VMA_MAYWRITE_BIT is checked instead of VMA_WRITE_BIT to account for both + * future mprotect() calls which can render a read-only mapping writable, and + * GUP with FOLL_FORCE (e.g. ptrace) which can CoW a read-only mapping. + * + * - No anonymous mapping can ever clear VMA_MAYWRITE_BIT. + * + * - Writes to anonymous mappings do not immediately result in CoW faults but + * may do so after the process is forked or if a read is followed by a + * write. + * + * - Writes to MAP_PRIVATE file-backed mappings result in CoW faults and may + * do so again after fork. + * + * - MAP_SHARED mappings of a file opened read-only are transformed into + * VMA_MAYSHARE_BIT, !VMA_SHARED_BIT, !VMA_MAYWRITE_BIT mappings, so remain + * non-CoW. + * + * - Drivers may clear VMA_MAYWRITE_BIT but do so at mmap() time and cannot + * mark themselves anonymous. Having cleared this flag it is not valid for + * them to leave the VMA_WRITE_BIT flag set. + * + * As a consequence, the anonymous reverse mapping only tracks CoW mappings. + * + * Returns: true if the flags indicate a CoW mapping, otherwise false. + */ static inline bool vma_flags_is_cow_mapping(const vma_flags_t *flags) { return vma_flags_test(flags, VMA_MAYWRITE_BIT) && !vma_flags_test(flags, VMA_SHARED_BIT); } +/** + * vma_is_cow_mapping() - Is this VMA a CoW mapping? + * @desc: The VMA to check. + * + * See vma_flags_is_cow_mapping() for details. + * + * Returns: true if the VMA is a CoW mapping, otherwise false. + */ static inline bool vma_is_cow_mapping(const struct vm_area_struct *vma) { return vma_flags_is_cow_mapping(&vma->flags); } +/** + * vma_desc_is_cow_mapping() - Is this VMA descriptor a CoW mapping? + * @desc: The VMA descriptor to check. + * + * See vma_flags_is_cow_mapping() for details. + * + * Returns: true if the VMA descriptor describes a CoW mapping, otherwise + * false. + */ static inline bool vma_desc_is_cow_mapping(struct vm_area_desc *desc) { return vma_flags_is_cow_mapping(&desc->vma_flags); _