Re: [PATCH v4 14/38] mm/vma: add vma[_flags]_is_mm_managed() predicates

[Suren Baghdasaryan]

On Sat, Oct 3, 2026 at 6:38 PM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
>
> Rather than referring to VMA flags with uncertain meaning, add a new
> predicate that explicitly describes what absence of the VMA_PFNMAP_BIT and
> VMA_MIXEDMAP_BIT flags means, and then refer to that function for
> determining VMA mergeability.
>
> If neither of these flags are set, that means the contents of the mapping
> are managed by core mm.
>
> Otherwise they are instead owned by some other kernel component, usually a
> driver: the memory may be MMIO, kernel-allocated pages or even ordinary
> pages the driver maps itself, but core mm must not populate, reclaim,
> migrate, copy-on-write or merge the range itself.
>
> We initially also treat VMA_IO_BIT as implying a mapping is not mm-managed,
> as by implication it cannot be. (mlock() also sets VMA_IO_BIT transiently
> on ordinary VMAs while locking them, which is addressed later in this
> series.)
>
> However the intent is to in future remove this, as no mapping should be
> marked as an I/O mapping without also being marked with VMA_PFNMAP_BIT.
>
> This forms the basis of further work intended to improve how we express VMA
> properties such as this.
>
> Also update the VMA userland tests to reflect the change.
>
> No functional change intended.
>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>


> ---
>  include/linux/mm.h              | 53 ++++++++++++++++++++++++++++++++++++++++-
>  tools/testing/vma/include/dup.h | 29 +++++++++++++++++++++-
>  2 files changed, 80 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 97e1eb4fe3a4..83953757735a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1612,6 +1612,41 @@ static inline bool vma_is_shared_maywrite(const struct vm_area_struct *vma)
>         return is_shared_maywrite(&vma->flags);
>  }
>
> +/**
> + * vma_flags_is_mm_managed() - Do the specified VMA flags indicate that the
> + * contents of the VMA are managed by core mm?
> + * @flags: The VMA flags to test.
> + *
> + * Such a mapping is one established by a simple mmap() or brk() with no driver
> + * or other kernel component involved.
> + *
> + * They are rmappable, and core mm services rmap operations including reclaim,
> + * as well as population, CoW and merge/split (if eligible).
> + *
> + * Mappings which fail this test are those where this is not the case -
> + * e.g. MMIO, user-mapped kernel pages, udmabuf (pfnmap shmem), etc.
> + *
> + * Returns: true if the flags indicate an mm-managed mapping.
> + */
> +static inline bool vma_flags_is_mm_managed(const vma_flags_t *flags)
> +{
> +       return !vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT,
> +                                  VMA_IO_BIT);
> +}
> +
> +/**
> + * vma_is_mm_managed() - Are the contents of @vma managed by core mm?
> + * @vma: The VMA to test.
> + *
> + * See vma_flags_is_mm_managed() for a description of this property.
> + *
> + * Returns: true if the VMA is mm-managed.
> + */
> +static inline bool vma_is_mm_managed(const struct vm_area_struct *vma)
> +{
> +       return vma_flags_is_mm_managed(&vma->flags);
> +}
> +
>  /**
>   * vma_flags_can_merge() - Do the specified VMA flags permit the VMA to be
>   * merged with another?
> @@ -1620,7 +1655,23 @@ static inline bool vma_is_shared_maywrite(const struct vm_area_struct *vma)
>   */
>  static inline bool vma_flags_can_merge(const vma_flags_t *flags)
>  {
> -       return !vma_flags_test_any_mask(flags, VMA_SPECIAL_FLAGS);
> +       /*
> +        * VMA merging assumes that a VMA's flags and fields completely describe
> +        * its state.
> +        *
> +        * However, mappings which are not mm-managed may have established state
> +        * upon mapping not embodied in any attribute of the VMA.
> +        *
> +        * Additionally, private (CoW) PFN maps encode the source PFN of the
> +        * range in vma->vm_pgoff, which may otherwise cause spurious merges.
> +        */
> +       if (!vma_flags_is_mm_managed(flags))
> +               return false;
> +       /* VMA explicitly marked as being unmergeable. */
> +       if (vma_flags_test(flags, VMA_DONTEXPAND_BIT))
> +               return false;
> +
> +       return true;
>  }
>
>  /**
> diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
> index a6a8e53c4b38..89c645a6ad60 100644
> --- a/tools/testing/vma/include/dup.h
> +++ b/tools/testing/vma/include/dup.h
> @@ -1665,7 +1665,34 @@ static inline bool file_is_dev_zero(const struct file *file)
>         return file && file->f_op == &zero_fops;
>  }
>
> +static inline bool vma_flags_is_mm_managed(const vma_flags_t *flags)
> +{
> +       return !vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT,
> +                                  VMA_IO_BIT);
> +}
> +
> +static inline bool vma_is_mm_managed(const struct vm_area_struct *vma)
> +{
> +       return vma_flags_is_mm_managed(&vma->flags);
> +}
> +
>  static inline bool vma_flags_can_merge(const vma_flags_t *flags)
>  {
> -       return !vma_flags_test_any_mask(flags, VMA_SPECIAL_FLAGS);
> +       /*
> +        * VMA merging assumes that a VMA's flags and fields completely describe
> +        * its state.
> +        *
> +        * However, mappings which are not mm-managed may have established state
> +        * upon mapping not embodied in any attribute of the VMA.
> +        *
> +        * Additionally, private (CoW) PFN maps encode the source PFN of the
> +        * range in vma->vm_pgoff, which may otherwise cause spurious merges.
> +        */
> +       if (!vma_flags_is_mm_managed(flags))
> +               return false;
> +       /* VMA explicitly marked as being unmergeable. */
> +       if (vma_flags_test(flags, VMA_DONTEXPAND_BIT))
> +               return false;
> +
> +       return true;
>  }
>
> --
> 2.55.0
>