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

Hi Andrew - would it be possible to insert the comments added below? Trying
to avoid a respin if possible :)

On Thu, Aug 06, 2026 at 09:21:27PM +0100, Lorenzo Stoakes (ARM) wrote:

 +/**
 + * 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 (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
+	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_desc_is_cow_mapping(struct vm_area_desc *desc)
+static inline bool vma_is_cow_mapping(const struct vm_area_struct *vma)
 {
-	const vma_flags_t *flags = &desc->vma_flags;
+	return vma_flags_is_cow_mapping(&vma->flags);
+}

-	return vma_flags_test(flags, VMA_MAYWRITE_BIT) &&
-		!vma_flags_test(flags, VMA_SHARED_BIT);

 +/**
 + * 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);
 }

 #ifndef CONFIG_MMU

--
Cheers, Lorenzo