Re: [RFC PATCH 02/11] mm, swap: add CONFIG_XSWAP and xswap fields to swap_info_struct

[Chris Li]

On Mon, Jul 27, 2026 at 7:05 AM Baoquan He <baoquan.he@xxxxxxxxx> wrote:
>
> Add CONFIG_XSWAP Kconfig option (depends on SWAP && 64BIT) for
> extendable (virtual) swap device support.
>
> Add three fields to struct swap_info_struct under CONFIG_XSWAP:
> - cluster_vm: the VM_SPARSE vm_struct backing the cluster_info array
> - nr_clusters: total number of clusters in the xswap address space
> - nr_clusters_mapped: number of clusters currently mapped (lazy grow)
>
> These fields enable lazy vmalloc-based dynamic cluster management
> where the cluster_info array is backed by a sparse vmalloc area that
> grows on demand and shrinks when clusters are freed.
>
> Signed-off-by: Baoquan He <baoquan.he@xxxxxxxxx>
> ---
>  include/linux/swap.h | 5 +++++
>  mm/Kconfig           | 9 +++++++++
>  2 files changed, 14 insertions(+)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 80c83dd8804f..5f6b14d30758 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -248,6 +248,11 @@ struct swap_info_struct {
>         signed char     type;           /* strange name for an index */
>         unsigned int    max;            /* size of this swap device */
>         struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
> +#ifdef CONFIG_XSWAP
> +       struct vm_struct        *cluster_vm;    /* VM_SPARSE area for xswap dynamic cluster_info */
> +       unsigned long           nr_clusters;    /* total cluster count for xswap */
> +       unsigned long           nr_clusters_mapped; /* currently mapped cluster count */
> +#endif

We might need more than one dynamic kvmalloc grow array. Maybe
abstract this into a struct so it can be reused for arrays other than
clusters as well. The abstraction can also include the element size so
it can have a generic function to calculate how many pages it needs
for N elements.

Chris