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Message-ID: <db2f17fd-6711-4521-8d2a-73c3fd8d914e@oracle.com>
Date: Mon, 8 Sep 2025 12:09:40 -0700
From: Anthony Yznaga <anthony.yznaga@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, linux-mm@...ck.org,
akpm@...ux-foundation.org, andreyknvl@...il.com, arnd@...db.de,
bp@...en8.de, brauner@...nel.org, bsegall@...gle.com, corbet@....net,
dave.hansen@...ux.intel.com, david@...hat.com,
dietmar.eggemann@....com, ebiederm@...ssion.com, hpa@...or.com,
jakub.wartak@...lbox.org, jannh@...gle.com, juri.lelli@...hat.com,
khalid@...nel.org, linyongting@...edance.com,
lorenzo.stoakes@...cle.com, luto@...nel.org, markhemm@...glemail.com,
maz@...nel.org, mhiramat@...nel.org, mgorman@...e.de, mhocko@...e.com,
mingo@...hat.com, muchun.song@...ux.dev, neilb@...e.de,
osalvador@...e.de, pcc@...gle.com, peterz@...radead.org,
pfalcato@...e.de, rostedt@...dmis.org, rppt@...nel.org,
shakeel.butt@...ux.dev, surenb@...gle.com, tglx@...utronix.de,
vasily.averin@...ux.dev, vbabka@...e.cz, vincent.guittot@...aro.org,
viro@...iv.linux.org.uk, vschneid@...hat.com, willy@...radead.org,
x86@...nel.org, xhao@...ux.alibaba.com, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 01/22] mm: Add msharefs filesystem
On 9/8/25 11:29 AM, Liam R. Howlett wrote:
> * 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?
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
>>
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