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>

1 files changed, 39 insertions, 0 deletions

diff --git a/include/linux/swap_ops.h b/include/linux/swap_ops.h
new file mode 100644
index 000000000000..e92b4f532604
--- /dev/null
+++ b/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 */