Re: [PATCH v4 02/20] mm: provide vma_[flags_]is_cow_mapping() and remove is_cow_mapping()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 10 Aug 2026 18:59:34 +0100 "Lorenzo Stoakes (ARM)" <ljs@kernel.org> 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 <akpm@linux-foundation.org>
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)" <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 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);
_