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:
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index df78847f5f07..a3368c542947 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2271,17 +2271,20 @@ void unpin_user_pages(struct page **pages, unsigned
> long npages);
> void unpin_user_folio(struct folio *folio, unsigned long npages);
> void unpin_folios(struct folio **folios, unsigned long nfolios);
>
> -static inline bool is_cow_mapping(vm_flags_t flags)
+/**
+ * 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
