Re: [PATCH v6 13/43] KVM: guest_memfd: Return early if range already has requested attributes

[Fuad Tabba]

On Thu, 7 May 2026 at 21:22, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
>
> Extract a helper out of kvm_gmem_range_is_private() that checks that a
> range has given attributes.
>
> Optimize setting memory attributes by returning early if all pages in the
> requested range already has the requested attributes.
>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad
> ---
>  virt/kvm/guest_memfd.c | 33 +++++++++++++++++++++++----------
>  1 file changed, 23 insertions(+), 10 deletions(-)
>
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index baf4b88dead1f..034b72b4947fb 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -86,6 +86,23 @@ static bool kvm_gmem_is_shared_mem(struct inode *inode, pgoff_t index)
>         return !kvm_gmem_is_private_mem(inode, index);
>  }
>
> +static bool kvm_gmem_range_has_attributes(struct maple_tree *mt,
> +                                         pgoff_t index, size_t nr_pages,
> +                                         u64 attributes)
> +{
> +       pgoff_t end = index + nr_pages - 1;
> +       void *entry;
> +
> +       lockdep_assert(mt_lock_is_held(mt));
> +
> +       mt_for_each(mt, entry, index, end) {
> +               if (xa_to_value(entry) != attributes)
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +
>  static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
>                                     pgoff_t index, struct folio *folio)
>  {
> @@ -649,12 +666,15 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start,
>         pgoff_t end = start + nr_pages;
>         struct maple_tree *mt;
>         struct ma_state mas;
> -       int r;
> +       int r = 0;
>
>         mt = &gi->attributes;
>
>         filemap_invalidate_lock(mapping);
>
> +       if (kvm_gmem_range_has_attributes(mt, start, nr_pages, attrs))
> +               goto out;
> +
>         mas_init(&mas, mt, start);
>         r = kvm_gmem_mas_preallocate(&mas, attrs, start, nr_pages);
>         if (r) {
> @@ -1140,20 +1160,13 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);
>  static bool kvm_gmem_range_is_private(struct gmem_inode *gi, pgoff_t index,
>                                       size_t nr_pages, struct kvm *kvm, gfn_t gfn)
>  {
> -       pgoff_t end = index + nr_pages - 1;
> -       void *entry;
> -
>         if (vm_memory_attributes)
>                 return kvm_range_has_vm_memory_attributes(kvm, gfn, gfn + nr_pages,
>                                                        KVM_MEMORY_ATTRIBUTE_PRIVATE,
>                                                        KVM_MEMORY_ATTRIBUTE_PRIVATE);
>
> -       mt_for_each(&gi->attributes, entry, index, end) {
> -               if (xa_to_value(entry) != KVM_MEMORY_ATTRIBUTE_PRIVATE)
> -                       return false;
> -       }
> -
> -       return true;
> +       return kvm_gmem_range_has_attributes(&gi->attributes, index, nr_pages,
> +                                            KVM_MEMORY_ATTRIBUTE_PRIVATE);
>  }
>
>  static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
>
> --
> 2.54.0.563.g4f69b47b94-goog
>
>