From: Khalid Aziz <khalid@kernel.org>

Add a pseudo filesystem that contains files and page table sharing
information that enables processes to share page table entries.
This patch adds the basic filesystem that can be mounted, a
CONFIG_MSHARE option to enable the feature, and documentation.

Signed-off-by: Khalid Aziz <khalid@kernel.org>
Signed-off-by: Anthony Yznaga <redacted>
---
 Documentation/filesystems/index.rst    |  1 +
 Documentation/filesystems/msharefs.rst | 96 +++++++++++++++++++++++++
 include/uapi/linux/magic.h             |  1 +
 mm/Kconfig                             | 11 +++
 mm/Makefile                            |  4 ++
 mm/mshare.c                            | 97 ++++++++++++++++++++++++++
 6 files changed, 210 insertions(+)
 create mode 100644 Documentation/filesystems/msharefs.rst
 create mode 100644 mm/mshare.c

diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 11a599387266..dcd6605eb228 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst

@@ -102,6 +102,7 @@ Documentation for filesystem implementations.
    fuse-passthrough
    inotify
    isofs
+   msharefs
    nilfs2
    nfs/index
    ntfs3

diff --git a/Documentation/filesystems/msharefs.rst b/Documentation/filesystems/msharefs.rst
new file mode 100644
index 000000000000..3e5b7d531821
--- /dev/null
+++ b/Documentation/filesystems/msharefs.rst

@@ -0,0 +1,96 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================================
+Msharefs - A filesystem to support shared page tables
+=====================================================
+
+What is msharefs?
+-----------------
+
+msharefs is a pseudo filesystem that allows multiple processes to
+share page table entries for shared pages. To enable support for
+msharefs the kernel must be compiled with CONFIG_MSHARE set.
+
+msharefs is typically mounted like this::
+
+	mount -t msharefs none /sys/fs/mshare
+
+A file created on msharefs creates a new shared region where all
+processes mapping that region will map it using shared page table
+entries. Once the size of the region has been established via
+ftruncate() or fallocate(), the region can be mapped into processes
+and ioctls used to map and unmap objects within it. Note that an
+msharefs file is a control file and accessing mapped objects within
+a shared region through read or write of the file is not permitted.
+
+How to use mshare
+-----------------
+
+Here are the basic steps for using mshare:
+
+  1. Mount msharefs on /sys/fs/mshare::
+
+	mount -t msharefs msharefs /sys/fs/mshare
+
+  2. mshare regions have alignment and size requirements. Start
+     address for the region must be aligned to an address boundary and
+     be a multiple of fixed size. This alignment and size requirement
+     can be obtained by reading the file ``/sys/fs/mshare/mshare_info``
+     which returns a number in text format. mshare regions must be
+     aligned to this boundary and be a multiple of this size.
+
+  3. For the process creating an mshare region:
+
+    a. Create a file on /sys/fs/mshare, for example::
+
+        fd = open("/sys/fs/mshare/shareme",
+                        O_RDWR|O_CREAT|O_EXCL, 0600);
+
+    b. Establish the size of the region::
+
+        fallocate(fd, 0, 0, BUF_SIZE);
+
+      or::
+
+        ftruncate(fd, BUF_SIZE);
+
+    c. Map some memory in the region::
+
+	struct mshare_create mcreate;
+
+	mcreate.region_offset = 0;
+	mcreate.size = BUF_SIZE;
+	mcreate.offset = 0;
+	mcreate.prot = PROT_READ | PROT_WRITE;
+	mcreate.flags = MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED;
+	mcreate.fd = -1;
+
+	ioctl(fd, MSHAREFS_CREATE_MAPPING, &mcreate);
+
+    d. Map the mshare region into the process::
+
+	mmap(NULL, BUF_SIZE,
+		PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+
+    e. Write and read to mshared region normally.
+
+
+  4. For processes attaching an mshare region:
+
+    a. Open the msharefs file, for example::
+
+	fd = open("/sys/fs/mshare/shareme", O_RDWR);
+
+    b. Get the size of the mshare region from the file::
+
+        fstat(fd, &sb);
+        mshare_size = sb.st_size;
+
+    c. Map the mshare region into the process::
+
+	mmap(NULL, mshare_size,
+		PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+
+  5. To delete the mshare region::
+
+		unlink("/sys/fs/mshare/shareme");

diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index bb575f3ab45e..e53dd6063cba 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h

@@ -103,5 +103,6 @@
 #define DEVMEM_MAGIC		0x454d444d	/* "DMEM" */
 #define SECRETMEM_MAGIC		0x5345434d	/* "SECM" */
 #define PID_FS_MAGIC		0x50494446	/* "PIDF" */
+#define MSHARE_MAGIC		0x4d534852	/* "MSHR" */
 
 #endif /* __LINUX_MAGIC_H__ */

diff --git a/mm/Kconfig b/mm/Kconfig
index 4108bcd96784..8b50e9785729 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig

@@ -1400,6 +1400,17 @@ config PT_RECLAIM
 config FIND_NORMAL_PAGE
 	def_bool n
 
+config MSHARE
+	bool "Mshare"
+	depends on MMU
+	help
+	  Enable msharefs: A pseudo filesystem that allows multiple processes
+	  to share kernel resources for mapping shared pages. A file created on
+	  msharefs represents a shared region where all processes mapping that
+	  region will map objects within it with shared page table entries and
+	  VMAs. Ioctls are used to configure and map objects into the shared
+	  region.
+
 source "mm/damon/Kconfig"
 
 endmenu

diff --git a/mm/Makefile b/mm/Makefile
index ef54aa615d9d..4af111b29c68 100644
--- a/mm/Makefile
+++ b/mm/Makefile

@@ -48,6 +48,10 @@ ifdef CONFIG_64BIT
 mmu-$(CONFIG_MMU)	+= mseal.o
 endif
 
+ifdef CONFIG_MSHARE
+mmu-$(CONFIG_MMU)	+= mshare.o
+endif
+
 obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
 			   maccess.o page-writeback.o folio-compat.o \
 			   readahead.o swap.o truncate.o vmscan.o shrinker.o \

diff --git a/mm/mshare.c b/mm/mshare.c
new file mode 100644
index 000000000000..f703af49ec81
--- /dev/null
+++ b/mm/mshare.c

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Enable cooperating 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) 2024 Oracle Corp. All rights reserved.
+ * Author:	Khalid Aziz <khalid@kernel.org>