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Message-ID: <20250820010415.699353-2-anthony.yznaga@oracle.com>
Date: Tue, 19 Aug 2025 18:03:54 -0700
From: Anthony Yznaga <anthony.yznaga@...cle.com>
To: linux-mm@...ck.org
Cc: akpm@...ux-foundation.org, andreyknvl@...il.com, arnd@...db.de,
bp@...en8.de, brauner@...nel.org, bsegall@...gle.com, corbet@....net,
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Subject: [PATCH v3 01/22] mm: Add msharefs filesystem
From: Khalid Aziz <khalid@...nel.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@...nel.org>
Signed-off-by: Anthony Yznaga <anthony.yznaga@...cle.com>
---
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@...nel.org>
+ *
+ */
+
+#include <linux/fs.h>
+#include <linux/fs_context.h>
+#include <uapi/linux/magic.h>
+
+static const struct file_operations msharefs_file_operations = {
+ .open = simple_open,
+};
+
+static const struct super_operations mshare_s_ops = {
+ .statfs = simple_statfs,
+};
+
+static int
+msharefs_fill_super(struct super_block *sb, struct fs_context *fc)
+{
+ struct inode *inode;
+
+ sb->s_blocksize = PAGE_SIZE;
+ sb->s_blocksize_bits = PAGE_SHIFT;
+ sb->s_maxbytes = MAX_LFS_FILESIZE;
+ sb->s_magic = MSHARE_MAGIC;
+ sb->s_op = &mshare_s_ops;
+ sb->s_time_gran = 1;
+
+ inode = new_inode(sb);
+ if (!inode)
+ return -ENOMEM;
+
+ inode->i_ino = 1;
+ inode->i_mode = S_IFDIR | 0777;
+ simple_inode_init_ts(inode);
+ inode->i_op = &simple_dir_inode_operations;
+ inode->i_fop = &simple_dir_operations;
+ set_nlink(inode, 2);
+
+ sb->s_root = d_make_root(inode);
+ if (!sb->s_root)
+ return -ENOMEM;
+
+ return 0;
+}
+
+static int
+msharefs_get_tree(struct fs_context *fc)
+{
+ return get_tree_nodev(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 __init
+mshare_init(void)
+{
+ int ret;
+
+ 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;
+}
+
+core_initcall(mshare_init);
--
2.47.1
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