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)" <[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?
Great thanks! Just one minor edit below (being pedantic here but kinda what the comment needs I think :) > > 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. 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
