On Mon, 10 Aug 2026 18:59:34 +0100 "Lorenzo Stoakes (ARM)" <[email protected]> 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 <[email protected]> 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)" <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- 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); _
