[PATCH v2 1/9] mm: Add msharefs filesystem

[Khalid Aziz <khalid.aziz@oracle.com>]

Add a ram-based filesystem that contains page table sharing
information and files that enables processes to share page tables.
This patch adds the basic filesystem that can be mounted.

Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
---
 Documentation/filesystems/msharefs.rst |  19 +++++
 include/uapi/linux/magic.h             |   1 +
 mm/Makefile                            |   2 +-
 mm/mshare.c                            | 103 +++++++++++++++++++++++++
 4 files changed, 124 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/filesystems/msharefs.rst
 create mode 100644 mm/mshare.c

diff --git a/Documentation/filesystems/msharefs.rst b/Documentation/filesystems/msharefs.rst
new file mode 100644
index 000000000000..fd161f67045d
--- /dev/null
+++ b/Documentation/filesystems/msharefs.rst
@@ -0,0 +1,19 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================================
+msharefs - a filesystem to support shared page tables
+=====================================================
+
+msharefs is a ram-based filesystem that allows multiple processes to
+share page table entries for shared pages.
+
+msharefs is typically mounted like this::
+
+	mount -t msharefs none /sys/fs/mshare
+
+When a process calls mshare syscall with a name for the shared address
+range, a file with the same name is created under msharefs with that
+name. This file can be opened by another process, if permissions
+allow, to query the addresses shared under this range. These files are
+removed by mshare_unlink syscall and can not be deleted directly.
+Hence these files are created as immutable files.
diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index f724129c0425..2a57a6ec6f3e 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -105,5 +105,6 @@
 #define Z3FOLD_MAGIC		0x33
 #define PPC_CMM_MAGIC		0xc7571590
 #define SECRETMEM_MAGIC		0x5345434d	/* "SECM" */
+#define MSHARE_MAGIC		0x4d534852	/* "MSHR" */
 
 #endif /* __LINUX_MAGIC_H__ */
diff --git a/mm/Makefile b/mm/Makefile
index 6f9ffa968a1a..51a2ab9080d9 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -37,7 +37,7 @@ CFLAGS_init-mm.o += $(call cc-disable-warning, override-init)
 CFLAGS_init-mm.o += $(call cc-disable-warning, initializer-overrides)
 
 mmu-y			:= nommu.o
-mmu-$(CONFIG_MMU)	:= highmem.o memory.o mincore.o \
+mmu-$(CONFIG_MMU)	:= highmem.o memory.o mincore.o mshare.o \
 			   mlock.o mmap.o mmu_gather.o mprotect.o mremap.o \
 			   msync.o page_vma_mapped.o pagewalk.o \
 			   pgtable-generic.o rmap.o vmalloc.o
diff --git a/mm/mshare.c b/mm/mshare.c
new file mode 100644
index 000000000000..c8fab3869bab
--- /dev/null
+++ b/mm/mshare.c
@@ -0,0 +1,103 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Enable copperating processes to share page table between
+ * them to reduce the extra memory consumed by multiple copies
+ * of page tables.
+ *
+ * This code adds an in-memory filesystem - msharefs.
+ * msharefs is used to manage page table sharing
+ *
+ *
+ * Copyright (C) 2022 Oracle Corp. All rights reserved.
+ * Author:	Khalid Aziz <khalid.aziz@oracle.com>
+ *
+ */
+
+#include <linux/fs.h>
+#include <linux/mount.h>
+#include <linux/syscalls.h>
+#include <linux/uaccess.h>
+#include <linux/pseudo_fs.h>
+#include <linux/fileattr.h>
+#include <uapi/linux/magic.h>
+#include <uapi/linux/limits.h>
+
+static struct super_block *msharefs_sb;
+
+static const struct file_operations msharefs_file_operations = {
+	.open	= simple_open,
+	.llseek	= no_llseek,
+};
+
+static int
+msharefs_d_hash(const struct dentry *dentry, struct qstr *qstr)
+{
+	unsigned long hash = init_name_hash(dentry);
+	const unsigned char *s = qstr->name;
+	unsigned int len = qstr->len;
+
+	while (len--)
+		hash = partial_name_hash(*s++, hash);
+	qstr->hash = end_name_hash(hash);
+	return 0;
+}
+
+static const struct dentry_operations msharefs_d_ops = {
+	.d_hash = msharefs_d_hash,
+};
+
+static int
+msharefs_fill_super(struct super_block *sb, struct fs_context *fc)
+{
+	static const struct tree_descr empty_descr = {""};
+	int err;
+
+	sb->s_d_op = &msharefs_d_ops;
+	err = simple_fill_super(sb, MSHARE_MAGIC, &empty_descr);
+	if (err)
+		return err;
+
+	msharefs_sb = sb;
+	return 0;
+}
+
+static int
+msharefs_get_tree(struct fs_context *fc)
+{
+	return get_tree_single(fc, msharefs_fill_super);
+}
+
+static const struct fs_context_operations msharefs_context_ops = {
+	.get_tree	= msharefs_get_tree,
+};
+
+static int
+mshare_init_fs_context(struct fs_context *fc)
+{
+	fc->ops = &msharefs_context_ops;
+	return 0;
+}
+
+static struct file_system_type mshare_fs = {
+	.name			= "msharefs",
+	.init_fs_context	= mshare_init_fs_context,
+	.kill_sb		= kill_litter_super,
+};
+
+static int
+mshare_init(void)
+{
+	int ret = 0;
+
+	ret = sysfs_create_mount_point(fs_kobj, "mshare");
+	if (ret)
+		return ret;
+
+	ret = register_filesystem(&mshare_fs);
+	if (ret)
+		sysfs_remove_mount_point(fs_kobj, "mshare");
+
+	return ret;
+}
+
+fs_initcall(mshare_init);
-- 
2.32.0