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

[Anthony Yznaga <anthony.yznaga@oracle.com>]

On 9/8/25 11:29 AM, Liam R. Howlett wrote:
> * 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?

Yes. I'll make sure the next series is updated.

> 
>> + *
>> + */
>> +
>> +#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?

If d_make_root() returns NULL it will have called iput_final(inode) 
which takes care of freeing the inode.

> 
>> +
>> +	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
>>