Re: [PATCH v3 3/9] KVM: guest_memfd: implement folio migration for non-confidential VMs

[David Hildenbrand (Arm)]

On 8/5/26 08:40, Shivank Garg wrote:
> Implement kvm_gmem_migrate_folio() using filemap_migrate_folio() for
> non-confidential VMs but don't enable it yet. guest_memfd mappings
> are still marked unmovable, so MM never calls it.
> 
> MM invokes ->migrate_folio() callback with folios already locked and
> acquires the invalidate lock afterwards. kvm_gmem_punch_hole() takes
> the invalidate lock before locking folios during truncation, so
> taking invalidate lock in migration callback can cause an ABBA-deadlock
> situation. Use filemap_invalidate_trylock_shared() to avoid this.
> 
> A subsequent patch adds an opt-in flag to change the mapping to
> movable and enable migration. Non-confidential VMs can use
> host-side copy via folio_mc_copy(), whereas confidential VMs will
> require firmware-assisted copying before they can opt in.
> 
> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
> ---
>  virt/kvm/guest_memfd.c | 36 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 169f75f95433..583341d593c9 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -488,13 +488,45 @@ static struct file_operations kvm_gmem_fops = {
>  	.fallocate	= kvm_gmem_fallocate,
>  };
>  
> +#ifdef CONFIG_MIGRATION
>  static int kvm_gmem_migrate_folio(struct address_space *mapping,
>  				  struct folio *dst, struct folio *src,
>  				  enum migrate_mode mode)
>  {
> -	WARN_ON_ONCE(1);
> -	return -EINVAL;
> +	struct inode *inode = mapping->host;
> +	pgoff_t start, end;
> +	int ret;
> +
> +	/*
> +	 * Migration invokes ->migrate_folio() while holding the folio lock.
> +	 * Use a non-blocking trylock to avoid inverting the lock order with
> +	 * truncation, which takes the invalidate lock before locking the
> +	 * folios.
> +	 */
> +	if (!filemap_invalidate_trylock_shared(mapping))
> +		return -EAGAIN;
> +
> +	start = src->index;
> +	end = start + folio_nr_pages(src);
> +
> +	kvm_gmem_invalidate_start(inode, start, end);
> +
> +	/*
> +	 * For non-confidential guests the folio is host-readable, so
> +	 * filemap_migrate_folio() can copy the contents itself via
> +	 * folio_mc_copy().
> +	 * For confidential guests, this would need firmware assistance.
> +	 */

I think this is all more closely related to mapping_inaccessible().

If accessible, this is just trivially true that we can support this.

So I would not comment on what to do for confidential VMs for now, and limit it
only to the shared-only case where pages are accessible.

-- 
Cheers,

David