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

On Mon, Aug 10, 2026 at 03:10:07PM -0700, Andrew Morton wrote:

On Mon, 10 Aug 2026 18:59:34 +0100 "Lorenzo Stoakes (ARM)" [off-list ref] wrote:

quoted

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

np, like this?

Great thanks!

Just one minor edit below (being pedantic here but kinda what the comment
needs I think :)

I ummed and ahhed over how to put this but now I think this could cause
confusion so could you replace the paragraph with:

 * All other mappings (including all MAP_SHARED mappings) are non-CoW.

It's _all_ MAP_SHARED mappings not just writable, I thought people would
automatically assume read-only wasn't included plus drivers could in theory
do crazy things (I have a pending series to fix that though) but it's
clearer just to say all.

+ *
+ * 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);
_

--
Cheers, Lorenzo