Re: [PATCH v3 01/22] mm: Add msharefs filesystem

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Anthony Yznaga <anthony.yznaga@oracle.com> [250819 21:04]:
> 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 <anthony.yznaga@oracle.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@kernel.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
>