[RFC 01/18] pkernfs: Introduce filesystem skeleton

[James Gowans <jgowans@amazon.com>]

Add an in-memory filesystem: pkernfs. Memory is donated to pkernfs by
carving it out of the normal System RAM range with the memmap= cmdline
parameter and then giving that same physical range to pkernfs with the
pkernfs= cmdline parameter.

A new filesystem is added; so far it doesn't do much except persist a
super block at the start of the donated memory and allows itself to be
mounted.
---
 fs/Kconfig           |  1 +
 fs/Makefile          |  3 ++
 fs/pkernfs/Kconfig   |  9 ++++
 fs/pkernfs/Makefile  |  6 +++
 fs/pkernfs/pkernfs.c | 99 ++++++++++++++++++++++++++++++++++++++++++++
 fs/pkernfs/pkernfs.h |  6 +++
 6 files changed, 124 insertions(+)
 create mode 100644 fs/pkernfs/Kconfig
 create mode 100644 fs/pkernfs/Makefile
 create mode 100644 fs/pkernfs/pkernfs.c
 create mode 100644 fs/pkernfs/pkernfs.h

diff --git a/fs/Kconfig b/fs/Kconfig
index aa7e03cc1941..33a9770ae657 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -331,6 +331,7 @@ source "fs/sysv/Kconfig"
 source "fs/ufs/Kconfig"
 source "fs/erofs/Kconfig"
 source "fs/vboxsf/Kconfig"
+source "fs/pkernfs/Kconfig"
 
 endif # MISC_FILESYSTEMS
 
diff --git a/fs/Makefile b/fs/Makefile
index f9541f40be4e..1af35b494b5d 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -19,6 +19,9 @@ obj-y :=	open.o read_write.o file_table.o super.o \
 
 obj-$(CONFIG_BUFFER_HEAD)	+= buffer.o mpage.o
 obj-$(CONFIG_PROC_FS)		+= proc_namespace.o
+
+obj-y += pkernfs/
+
 obj-$(CONFIG_LEGACY_DIRECT_IO)	+= direct-io.o
 obj-y				+= notify/
 obj-$(CONFIG_EPOLL)		+= eventpoll.o
diff --git a/fs/pkernfs/Kconfig b/fs/pkernfs/Kconfig
new file mode 100644
index 000000000000..59621a1d9aef
--- /dev/null
+++ b/fs/pkernfs/Kconfig
@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config PKERNFS_FS
+	bool "Persistent Kernel filesystem (pkernfs)"
+	help
+	  An in-memory filesystem on top of reserved memory specified via
+	  pkernfs= cmdline argument.  Used for storing kernel state and
+	  userspace memory which is preserved across kexec to support
+	  live update.
diff --git a/fs/pkernfs/Makefile b/fs/pkernfs/Makefile
new file mode 100644
index 000000000000..17258cb77f58
--- /dev/null
+++ b/fs/pkernfs/Makefile
@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Makefile for persistent kernel filesystem
+#
+
+obj-$(CONFIG_PKERNFS_FS) += pkernfs.o
diff --git a/fs/pkernfs/pkernfs.c b/fs/pkernfs/pkernfs.c
new file mode 100644
index 000000000000..4c476ddc35b6
--- /dev/null
+++ b/fs/pkernfs/pkernfs.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include "pkernfs.h"
+#include <linux/dcache.h>
+#include <linux/fs.h>
+#include <linux/module.h>
+#include <linux/fs_context.h>
+#include <linux/io.h>
+
+static phys_addr_t pkernfs_base, pkernfs_size;
+static void *pkernfs_mem;
+static const struct super_operations pkernfs_super_ops = { };
+
+static int pkernfs_fill_super(struct super_block *sb, struct fs_context *fc)
+{
+	struct inode *inode;
+	struct dentry *dentry;
+	struct pkernfs_sb *psb;
+
+	pkernfs_mem = memremap(pkernfs_base, pkernfs_size, MEMREMAP_WB);
+	psb = (struct pkernfs_sb *) pkernfs_mem;
+
+	if (psb->magic_number == PKERNFS_MAGIC_NUMBER) {
+		pr_info("pkernfs: Restoring from super block\n");
+	} else {
+		pr_info("pkernfs: Clean super block; initialising\n");
+		psb->magic_number = PKERNFS_MAGIC_NUMBER;
+	}
+
+	sb->s_op = &pkernfs_super_ops;
+
+	inode = new_inode(sb);
+	if (!inode)
+		return -ENOMEM;
+
+	inode->i_ino = 1;
+	inode->i_mode = S_IFDIR;
+	inode->i_op = &simple_dir_inode_operations;
+	inode->i_fop = &simple_dir_operations;
+	inode->i_atime = inode->i_mtime = current_time(inode);
+	inode_set_ctime_current(inode);
+	/* directory inodes start off with i_nlink == 2 (for "." entry) */
+	inc_nlink(inode);
+
+	dentry = d_make_root(inode);
+	if (!dentry)
+		return -ENOMEM;
+	sb->s_root = dentry;
+
+	return 0;
+}
+
+static int pkernfs_get_tree(struct fs_context *fc)
+{
+	return get_tree_nodev(fc, pkernfs_fill_super);
+}
+
+static const struct fs_context_operations pkernfs_context_ops = {
+	.get_tree	= pkernfs_get_tree,
+};
+
+static int pkernfs_init_fs_context(struct fs_context *const fc)
+{
+	fc->ops = &pkernfs_context_ops;
+	return 0;
+}
+
+static struct file_system_type pkernfs_fs_type = {
+	.owner                  = THIS_MODULE,
+	.name                   = "pkernfs",
+	.init_fs_context        = pkernfs_init_fs_context,
+	.kill_sb                = kill_litter_super,
+	.fs_flags               = FS_USERNS_MOUNT,
+};
+
+static int __init pkernfs_init(void)
+{
+	int ret;
+
+	ret = register_filesystem(&pkernfs_fs_type);
+	return ret;
+}
+
+/**
+ * Format: pkernfs=<size>:<base>
+ * Just like: memmap=nn[KMG]!ss[KMG]
+ */
+static int __init parse_pkernfs_extents(char *p)
+{
+	pkernfs_size = memparse(p, &p);
+	p++; /* Skip over ! char */
+	pkernfs_base = memparse(p, &p);
+	return 0;
+}
+
+early_param("pkernfs", parse_pkernfs_extents);
+
+MODULE_ALIAS_FS("pkernfs");
+module_init(pkernfs_init);
diff --git a/fs/pkernfs/pkernfs.h b/fs/pkernfs/pkernfs.h
new file mode 100644
index 000000000000..bd1e2a6fd336
--- /dev/null
+++ b/fs/pkernfs/pkernfs.h
@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#define PKERNFS_MAGIC_NUMBER 0x706b65726e6673
+struct pkernfs_sb {
+	unsigned long magic_number;
+};
-- 
2.40.1