[RFC PATCH 03/10] mm/swap: add redirect_xa field and ghost redirect helper declarations

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

Add a 'struct xarray redirect_xa' to swap_info_struct (guarded by
CONFIG_ZSWAP) to persist ghost_offset -> phys_swp_entry_t mappings
across swap-cache PFN cycles.

Declare the ghost redirect helper functions in mm/swap.h.
All are no-ops when CONFIG_ZSWAP=n.

Signed-off-by: Baoquan He <baoquan.he@linux.dev>
---
 include/linux/swap.h |  3 +++
 mm/swap.h            | 14 ++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 9829a31a541e..a4d6851e0921 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -277,6 +277,9 @@ struct swap_info_struct {
 	struct work_struct reclaim_work; /* reclaim worker */
 	struct list_head discard_clusters; /* discard clusters list */
 	struct plist_node avail_list;   /* entry in swap_avail_head */
+#ifdef CONFIG_ZSWAP
+	struct xarray redirect_xa;	/* ghost offset → phys swp_entry_t */
+#endif
 };
 
 static inline swp_entry_t page_swap_entry(struct page *page)
diff --git a/mm/swap.h b/mm/swap.h
index 2d29e41d3323..bc51ee5dbf51 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -287,6 +287,20 @@ void swap_write_unplug(struct swap_iocb *sio);
 int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug);
 int __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
 
+/* Ghost swapfile redirect helpers (mm/swapfile.c) */
+#ifdef CONFIG_ZSWAP
+swp_entry_t ghost_redirect_to_phys(struct swap_info_struct *si,
+				    pgoff_t offset);
+swp_entry_t alloc_phys_swap_slot(void);
+void free_phys_swap_slot(swp_entry_t phys_entry);
+#else
+static inline swp_entry_t ghost_redirect_to_phys(struct swap_info_struct *si,
+						  pgoff_t offset)
+{ return (swp_entry_t){}; }
+static inline swp_entry_t alloc_phys_swap_slot(void) { return (swp_entry_t){}; }
+static inline void free_phys_swap_slot(swp_entry_t phys_entry) {}
+#endif
+
 /* linux/mm/swap_state.c */
 extern struct address_space swap_space __read_mostly;
 static inline struct address_space *swap_address_space(swp_entry_t entry)
-- 
2.54.0