Re: [PATCH v4 01/20] mm/vma: introduce VMA anon page offset field and add helpers

On Thu, Aug 6, 2026 at 1:22 PM Lorenzo Stoakes (ARM) <[email protected]> wrote:
>
> Establish fields in vm_area_struct to store the anonymous page offset of
> VMAs.
>
> Initially, the anonymous page offset of a VMA is vma->vm_start >>
> PAGE_SHIFT.
>
> When a VMA is remapped to new_address its anonymous page offset is either
> updated to new_address >> PAGE_SHIFT if unfaulted or, if faulted, remains
> equal to the anonymous page offset it had when first faulted.
>
> Currently, anonymous folios belonging to CoW'd MAP_PRIVATE-mapped
> file-backed VMAs are tracked by their file offsets. By adding anonymous
> offset as a property of VMAs, we can now track them by their anonymous page
> offset instead.
>
> By tracking this, we provide the means by which to eliminate this
> inconsistency, and more importantly lay the foundations for future work for
> the scalable CoW anonymous rmap rework.
>
> This patch simply adds the fields and some simple helpers. Subsequent
> patches will update mm code to make use of these fields correctly.
>
> The fields chosen are packed in the VMA such that, for 64-bit kernel
> builds, no additional space is taken up.
>
> The first field is present on cacheline 0 containing key VMA fields, and
> the second on cacheline 3, which contains file-backed reverse mapping
> fields.
>
> Given the relative time spent accessing reverse mapping fields as well as
> updating them, there shouldn't be any performance impact here from false
> sharing.
>
> Update the VMA userland tests to account for this change.
>
> No callsites are updated yet, so no functional change intended.
>
> Acked-by: David Hildenbrand (Arm) <[email protected]>
> Reviewed-by: Gregory Price (Meta) <[email protected]>
> Reviewed-by: Xu Xin <[email protected]>
> Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
> ---
>  include/linux/mm.h              | 59 +++++++++++++++++++++++++++++++++++++++++
>  include/linux/mm_types.h        | 12 +++++++++
>  mm/vma.h                        | 14 ++++++++++
>  mm/vma_init.c                   |  1 +
>  tools/testing/vma/include/dup.h | 26 ++++++++++++++++++
>  5 files changed, 112 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 87feaa5a2b78..df78847f5f07 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4393,6 +4393,65 @@ static inline pgoff_t vma_last_pgoff(const struct vm_area_struct *vma)
>         return vma_end_pgoff(vma) - 1;
>  }
>
> +/**
> + * vma_start_anon_pgoff() - Get the anonymous page offset of the start of @vma
> + * @vma: The VMA whose anonymous page offset is required.
> + *
> + * If unfaulted, then this is vma->vm_start >> PAGE_SHIFT, if faulted then the
> + * anonymous page offset at the time of first fault.
> + *
> + * If the VMA is anonymous, this returns the same value as vma_start_pgoff().
> + *
> + * This value is used for tracking MAP_PRIVATE file-backed mappings by their
> + * anonymous page offset.

I assume this function should not be used with shared file-backed
mappings, right? If so, maybe add a comment like the one you have for
linear_anon_page_index(): "It is not valid to call this function for
shared file-backed mappings."?



> + *
> + * Returns: The anonymous page offset of the start of @vma.
> + */
> +static inline pgoff_t vma_start_anon_pgoff(const struct vm_area_struct *vma)
> +{
> +       pgoff_t pgoff = 0;
> +
> +#ifdef CONFIG_64BIT
> +       pgoff += vma->__vm_anon_pgoff_hi;
> +       pgoff <<= 32;
> +#endif
> +       pgoff += vma->__vm_anon_pgoff_lo;
> +       return pgoff;
> +}
> +
> +/**
> + * vma_end_anon_pgoff() - Get the anonymous page offset of the exclusive end of
> + * @vma.
> + * @vma: The VMA whose end anonymous page offset is required.
> + *
> + * This returns the anonymous exclusive end page offset of @vma, which is useful
> + * for expressing page offset ranges.
> + *
> + * See the description of vma_start_anon_pgoff() for a description of VMA
> + * anonymous page offsets.
> + *
> + * Returns: The exclusive end anonymous page offset of @vma.
> + */
> +static inline pgoff_t vma_end_anon_pgoff(const struct vm_area_struct *vma)
> +{
> +       return vma_start_anon_pgoff(vma) + vma_pages(vma);
> +}
> +
> +/**
> + * vma_last_anon_pgoff() - Get the anonymous page offset of the last page in
> + * @vma.
> + * @vma: The VMA whose last anonymous page offset is required.
> + *
> + * See the description of vma_start_anon_pgoff() for a description of VMA
> + * anonymous page offsets.
> + *
> + * Returns: The last anonymous page offset of @vma.
> + */
> +static inline pgoff_t vma_last_anon_pgoff(const struct vm_area_struct *vma)
> +{
> +       return vma_end_anon_pgoff(vma) - 1;
> +}
> +
>  static inline unsigned long vma_desc_size(const struct vm_area_desc *desc)
>  {
>         return desc->end - desc->start;
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 939b5ea8c9e0..ebf0d912be7d 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -967,6 +967,11 @@ struct vm_area_struct {
>          */
>         unsigned int vm_lock_seq;
>  #endif
> +       /*
> +        * Low 32-bits of anonymous page offset.
> +        * See vma_start_anon_pgoff() comment for details.
> +        */
> +       unsigned int __vm_anon_pgoff_lo;
>         /*
>          * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
>          * list, after a COW of one of the file pages.  A MAP_SHARED vma
> @@ -1041,6 +1046,13 @@ struct vm_area_struct {
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>         struct lockdep_map vmlock_dep_map;
>  #endif
> +#endif
> +#ifdef CONFIG_64BIT
> +       /*
> +        * High 32-bits of anonymous page offset.
> +        * See vma_start_anon_pgoff() comment for details.
> +        */
> +       unsigned int __vm_anon_pgoff_hi;
>  #endif
>         /*
>          * For areas with an address space and backing store,
> diff --git a/mm/vma.h b/mm/vma.h
> index 0bc7d521e976..54ed7c744e3b 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -283,6 +283,20 @@ static inline void vma_set_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
>         vma->vm_pgoff = pgoff;
>  }
>
> +static inline void __vma_set_anon_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
> +{
> +#ifdef CONFIG_64BIT
> +       vma->__vm_anon_pgoff_hi = pgoff >> 32;
> +#endif
> +       vma->__vm_anon_pgoff_lo = pgoff & GENMASK(31, 0);
> +}
> +
> +static inline void vma_set_anon_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
> +{
> +       vma_assert_can_modify(vma);
> +       __vma_set_anon_pgoff(vma, pgoff);
> +}
> +
>  static inline void vma_add_pgoff(struct vm_area_struct *vma, pgoff_t delta)
>  {
>         vma_assert_can_modify(vma);
> diff --git a/mm/vma_init.c b/mm/vma_init.c
> index 715feee283f0..baa7e82f47e3 100644
> --- a/mm/vma_init.c
> +++ b/mm/vma_init.c
> @@ -51,6 +51,7 @@ static void vm_area_init_from(const struct vm_area_struct *src,
>         dest->vm_end = src->vm_end;
>         dest->anon_vma = src->anon_vma;
>         dest->vm_pgoff = vma_start_pgoff(src);
> +       __vma_set_anon_pgoff(dest, vma_start_anon_pgoff(src));
>         dest->vm_file = src->vm_file;
>         dest->vm_private_data = src->vm_private_data;
>         vm_flags_init(dest, src->vm_flags);
> diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
> index c52b23773cd2..26d9210b1e8b 100644
> --- a/tools/testing/vma/include/dup.h
> +++ b/tools/testing/vma/include/dup.h
> @@ -577,6 +577,7 @@ struct vm_area_struct {
>          */
>         unsigned int vm_lock_seq;
>  #endif
> +       unsigned int __vm_anon_pgoff_lo;
>
>         /*
>          * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
> @@ -612,6 +613,9 @@ struct vm_area_struct {
>  #ifdef CONFIG_PER_VMA_LOCK
>         /* Unstable RCU readers are allowed to read this. */
>         refcount_t vm_refcnt;
> +#endif
> +#ifdef CONFIG_64BIT
> +       unsigned int __vm_anon_pgoff_hi;
>  #endif
>         /*
>          * For areas with an address space and backing store,
> @@ -1320,6 +1324,28 @@ static inline pgoff_t vma_end_pgoff(const struct vm_area_struct *vma)
>         return vma_start_pgoff(vma) + vma_pages(vma);
>  }
>
> +static inline pgoff_t vma_start_anon_pgoff(const struct vm_area_struct *vma)
> +{
> +       pgoff_t pgoff = 0;
> +
> +#ifdef CONFIG_64BIT
> +       pgoff += vma->__vm_anon_pgoff_hi;
> +       pgoff <<= 32;
> +#endif
> +       pgoff += vma->__vm_anon_pgoff_lo;
> +       return pgoff;
> +}
> +
> +static inline pgoff_t vma_end_anon_pgoff(const struct vm_area_struct *vma)
> +{
> +       return vma_start_anon_pgoff(vma) + vma_pages(vma);
> +}
> +
> +static inline pgoff_t vma_last_anon_pgoff(const struct vm_area_struct *vma)
> +{
> +       return vma_end_anon_pgoff(vma) - 1;
> +}
> +
>  static inline int vfs_mmap_prepare(struct file *file, struct vm_area_desc *desc)
>  {
>         return file->f_op->mmap_prepare(desc);
>
> --
> 2.55.0
>