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Message-ID: <lgwf5d2y2ok2f52l7xi6klln4hbncnoyn2s2jdi3k4wi5ngfz4@kbtqjwirdnqj>
Date: Mon, 8 Sep 2025 14:29:55 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Anthony Yznaga <anthony.yznaga@...cle.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org, andreyknvl@...il.com,
arnd@...db.de, bp@...en8.de, brauner@...nel.org, bsegall@...gle.com,
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Subject: Re: [PATCH v3 01/22] mm: Add msharefs filesystem
* Anthony Yznaga <anthony.yznaga@...cle.com> [250819 21:04]:
> 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>
Probably needs a new year or year range and another author?
> + *
> + */
> +
> +#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;
I don't know the recovery here, but what about inode and inode link
count?
> +
> + 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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