In prepration for the implementation of swap virtualization, add new
scaffolds for the new code: a new mm/vswap.c source file, which
currently only holds the logic to set up the (for now, empty) vswap
debugfs directory. Hook this up in the swap setup step in
mm/swap_state.c, and set up vswap compilation in the Makefile.

Other than the debugfs directory, no behavioral change intended.

Finally, make Johannes a swap reviewer, given that he has contributed
majorly to the developments of virtual swap.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 MAINTAINERS          |  2 ++
 include/linux/swap.h |  3 +++
 mm/Makefile          |  2 +-
 mm/swap_state.c      |  6 ++++++
 mm/vswap.c           | 35 +++++++++++++++++++++++++++++++++++
 5 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 mm/vswap.c

diff --git a/MAINTAINERS b/MAINTAINERS
index d3780bb330378..042dbc06c3d3c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16728,6 +16728,7 @@ R:	Kemeng Shi <shikemeng@huaweicloud.com>
 R:	Nhat Pham <nphamcs@gmail.com>
 R:	Baoquan He <bhe@redhat.com>
 R:	Barry Song <baohua@kernel.org>
+R:	Johannes Weiner <hannes@cmpxchg.org>
 L:	linux-mm@kvack.org
 S:	Maintained
 F:	include/linux/swap.h
@@ -16739,6 +16740,7 @@ F:	mm/swap.h
 F:	mm/swap_table.h
 F:	mm/swap_state.c
 F:	mm/swapfile.c
+F:	mm/vswap.c
 
 MEMORY MANAGEMENT - THP (TRANSPARENT HUGE PAGE)
 M:	Andrew Morton <akpm@linux-foundation.org>
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 918b47da55f44..df0771903a952 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -423,6 +423,9 @@ extern void __meminit kswapd_stop(int nid);
 
 #ifdef CONFIG_SWAP
 
+/* Virtual swap space API (mm/vswap.c) */
+int vswap_init(void);
+
 /* Lifecycle swap API (mm/swapfile.c) */
 int folio_alloc_swap(struct folio *folio);
 bool folio_free_swap(struct folio *folio);
diff --git a/mm/Makefile b/mm/Makefile
index 2d0570a16e5be..67fa4586e7e18 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -75,7 +75,7 @@ ifdef CONFIG_MMU
 	obj-$(CONFIG_ADVISE_SYSCALLS)	+= madvise.o
 endif
 
-obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o
+obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o vswap.o
 obj-$(CONFIG_ZSWAP)	+= zswap.o
 obj-$(CONFIG_HAS_DMA)	+= dmapool.o
 obj-$(CONFIG_HUGETLBFS)	+= hugetlb.o hugetlb_sysfs.o hugetlb_sysctl.o
diff --git a/mm/swap_state.c b/mm/swap_state.c
index e2e9f55bea3bb..29ec666be4204 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -882,6 +882,12 @@ static int __init swap_init(void)
 	int err;
 	struct kobject *swap_kobj;
 
+	err = vswap_init();
+	if (err) {
+		pr_err("failed to initialize virtual swap space\n");
+		return err;
+	}
+
 	swap_kobj = kobject_create_and_add("swap", mm_kobj);
 	if (!swap_kobj) {
 		pr_err("failed to create swap kobject\n");
diff --git a/mm/vswap.c b/mm/vswap.c
new file mode 100644
index 0000000000000..e68234f053fc9
--- /dev/null
+++ b/mm/vswap.c
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Virtual swap space
+ *
+ * Copyright (C) 2024 Meta Platforms, Inc., Nhat Pham
+ */
+#include <linux/swap.h>
+
+#ifdef CONFIG_DEBUG_FS
+#include <linux/debugfs.h>
+
+static struct dentry *vswap_debugfs_root;
+
+static int vswap_debug_fs_init(void)
+{
+	if (!debugfs_initialized())
+		return -ENODEV;
+
+	vswap_debugfs_root = debugfs_create_dir("vswap", NULL);
+	return 0;
+}
+#else
+static int vswap_debug_fs_init(void)
+{
+	return 0;
+}
+#endif
+
+int vswap_init(void)
+{
+	if (vswap_debug_fs_init())
+		pr_warn("Failed to initialize vswap debugfs\n");
+
+	return 0;
+}
-- 
2.52.0