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

[Baoquan He <baoquan.he@linux.dev>]

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@linux.dev>
---
 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
 	struct list_head free_clusters; /* free clusters list */
 	struct list_head full_clusters; /* full clusters list */
 	struct list_head nonfull_clusters[SWAP_NR_ORDERS];
diff --git a/mm/Kconfig b/mm/Kconfig
index c52ab6afcb15..889d5ce20a55 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -122,6 +122,15 @@ config ZSWAP_COMPRESSOR_DEFAULT
        default "zstd" if ZSWAP_COMPRESSOR_DEFAULT_ZSTD
        default ""
 
+config XSWAP
+	bool "Extendable (virtual) swap device"
+	depends on SWAP && 64BIT
+	help
+	  Adds support for extendable swap devices (xswap) that decouple
+	  PTE swap entries from physical backing storage. The cluster_info
+	  array is backed by a sparse vmalloc area that grows and shrinks
+	  on demand, avoiding static pre-allocation overhead.
+
 config ZSMALLOC
 	tristate
 
-- 
2.54.0