mm/swap: add a new swap_ops.h header to allow for pluggable swap ops

Add a new header to declare the swap_iocb, swap_ops and swap_ctx to allow
for swap_ops implementations outside of mm/page_io.c.  This will be used
to remove the double indirection for file system-based swap.  There is no
functional change, just a move of the declarations.

Note that there already is a swapops.h header, which is totally unrelated
to struct swap_ops.  The close naming is a bit unfortunate, but I could
not think of a better name for this header.

Link: https://lore.kernel.org/20260723054622.3460249-3-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Steve French <sfrench@samba.org>
Cc: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

MAINTAINERS

@@ -17242,6 +17242,7 @@ S:	Maintained
 F:	Documentation/ABI/testing/sysfs-kernel-mm-swap
 F:	Documentation/mm/swap-table.rst
 F:	include/linux/swap.h
+F:	include/linux/swap_ops.h
 F:	include/linux/swapfile.h
 F:	include/linux/swapops.h
 F:	mm/page_io.c

include/linux/swap_ops.h

@@ -0,0 +1,39 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _MM_SWAP_OPS_H
+#define _MM_SWAP_OPS_H
+
+#include <linux/swap.h> /* for SWAP_CLUSTER_MAX */
+
+struct swap_iocb {
+	union {
+		struct kiocb	iocb;
+		struct bio	bio;
+	};
+	struct bio_vec		bvecs[SWAP_CLUSTER_MAX];
+	int			nr_bvecs;
+	int			len;
+};
+
+struct swap_io_ctx {
+	struct swap_iocb	*sio;
+	struct swap_info_struct	*sis;
+};
+
+/*
+ * SWAP_OPS_F_REQUIRE_NOFS:
+ *	When set, all reclaim operations must operated as GFS_NOFS and not
+ *	just GFP_NOIO, as GFP_NOIO allocations could recourse into the
+ *	file system backing this swap file.
+ */
+#define SWAP_OPS_F_REQUIRE_NOFS		(1U << 0)
+
+struct swap_ops {
+	unsigned int		flags;
+
+	bool (*can_merge)(struct folio *folio, struct folio *prev_folio,
+			size_t prev_folio_size, int rw);
+	void (*submit_write)(struct swap_io_ctx *ctx);
+	void (*submit_read)(struct swap_io_ctx *ctx);
+};
+
+#endif /* _MM_SWAP_OPS_H */

mm/madvise.c

@@ -32,6 +32,7 @@
 #include <linux/leafops.h>
 #include <linux/shmem_fs.h>
 #include <linux/mmu_notifier.h>
+#include <linux/swap_ops.h>
 
 #include <asm/tlb.h>
 

mm/page_io.c

@@ -25,6 +25,7 @@
 #include <linux/sched/task.h>
 #include <linux/delayacct.h>
 #include <linux/zswap.h>
+#include <linux/swap_ops.h>
 #include "swap.h"
 #include "swap_table.h"
 
@@ -300,15 +301,6 @@ static bool folio_blkg_can_merge(struct folio *folio, struct folio *prev_folio)
 #define bio_associate_blkg_from_page(bio, folio)		do { } while (0)
 #endif /* CONFIG_MEMCG && CONFIG_BLK_CGROUP */
 
-struct swap_iocb {
-	union {
-		struct kiocb	iocb;
-		struct bio	bio;
-	};
-	struct bio_vec		bvecs[SWAP_CLUSTER_MAX];
-	int			nr_bvecs;
-	int			len;
-};
 static mempool_t *sio_pool;
 
 int sio_pool_init(void)

mm/shmem.c

@@ -41,6 +41,7 @@
 #include <linux/swapfile.h>
 #include <linux/iversion.h>
 #include <linux/unicode.h>
+#include <linux/swap_ops.h>
 #include "swap.h"
 
 static struct vfsmount *shm_mnt __ro_after_init;

mm/swap.h

@@ -10,6 +10,7 @@
 struct mempolicy;
 struct swap_iocb;
 struct swap_memcg_table;
+struct swap_io_ctx;
 
 #if defined(MAX_POSSIBLE_PHYSMEM_BITS)
 #define SWAP_CACHE_PFN_BITS (MAX_POSSIBLE_PHYSMEM_BITS - PAGE_SHIFT)
@@ -91,28 +92,6 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
 	return READ_ONCE(vm_swappiness);
 }
 
-struct swap_io_ctx {
-	struct swap_iocb	*sio;
-	struct swap_info_struct	*sis;
-};
-
-/*
- * SWAP_OPS_F_REQUIRE_NOFS:
- *	When set, all reclaim operations must operated as GFS_NOFS and not
- *	just GFP_NOIO, as GFP_NOIO allocations could recourse into the
- *	file system backing this swap file.
- */
-#define SWAP_OPS_F_REQUIRE_NOFS		(1U << 0)
-
-struct swap_ops {
-	unsigned int		flags;
-
-	bool (*can_merge)(struct folio *folio, struct folio *prev_folio,
-			size_t prev_folio_size, int rw);
-	void (*submit_write)(struct swap_io_ctx *ctx);
-	void (*submit_read)(struct swap_io_ctx *ctx);
-};
-
 #ifdef CONFIG_SWAP
 #include <linux/swapops.h> /* for swp_offset */
 #include <linux/blk_types.h> /* for bio_end_io_t */

mm/swap_state.c

@@ -23,6 +23,7 @@
 #include <linux/huge_mm.h>
 #include <linux/shmem_fs.h>
 #include <linux/sysctl.h>
+#include <linux/swap_ops.h>
 #include "internal.h"
 #include "swap_table.h"
 #include "swap.h"

mm/vmscan.c

@@ -58,6 +58,7 @@
 #include <linux/random.h>
 #include <linux/mmu_notifier.h>
 #include <linux/parser.h>
+#include <linux/swap_ops.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>

mm/zswap.c

@@ -20,7 +20,7 @@
 #include <linux/spinlock.h>
 #include <linux/types.h>
 #include <linux/atomic.h>
-#include <linux/swap.h>
+#include <linux/swap_ops.h>
 #include <linux/crypto.h>
 #include <linux/scatterlist.h>
 #include <linux/mempolicy.h>