Re: [PATCH 01/20] mm: Add msharefs filesystem

[Anthony Yznaga]

On 2/3/25 5:52 PM, Bagas Sanjaya wrote:
> On Fri, Jan 24, 2025 at 03:54:35PM -0800, Anthony Yznaga wrote:
> > diff --git a/Documentation/filesystems/msharefs.rst b/Documentation/filesystems/msharefs.rst
> > new file mode 100644
> > index 000000000000..c3c7168aa18f
> > --- /dev/null
> > +++ b/Documentation/filesystems/msharefs.rst
> > @@ -0,0 +1,107 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +=====================================================
> > +msharefs - a filesystem to support shared page tables
> > +=====================================================
> > +
> > +msharefs is a ram-based 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. ioctls are used to initialize or retrieve the start address
> > +and size of a shared region and to map objects in the shared
> > +region. It is important to note that an msharefs file is a control
> > +file for the shared region and does not contain the contents
> > +of the region itself.
> > +
> > +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:
> Should the creating mshare region sublist be nested list?
Can you expand on that? Do you mean create an mshare region as a 
directory and populate it with files representing the mappings that are 
created in the region?
> > +
> > +.. code-block:: c
> > +
> > +	fd = open("/sys/fs/mshare/shareme",
> > +			O_RDWR|O_CREAT|O_EXCL, 0600);
> > +
> > +b. Establish the starting address and size of the region:
> > +
> > +.. code-block:: c
> > +
> > +	struct mshare_info minfo;
> > +
> > +	minfo.start = TB(2);
> > +	minfo.size = BUFFER_SIZE;
> > +	ioctl(fd, MSHAREFS_SET_SIZE, &minfo);
> > +
> > +c. Map some memory in the region:
> > +
> > +.. code-block:: c
> > +
> > +	struct mshare_create mcreate;
> > +
> > +	mcreate.addr = TB(2);
> > +	mcreate.size = BUFFER_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:
> > +
> > +.. code-block:: c
> > +
> > +	mmap((void *)TB(2), 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 file on msharefs, for example:
> > +
> > +.. code-block:: c
> > +
> > +	fd = open("/sys/fs/mshare/shareme", O_RDWR);
> > +
> > +b. Get information about mshare'd region from the file:
> > +
> > +.. code-block:: c
> > +
> > +	struct mshare_info minfo;
> > +
> > +	ioctl(fd, MSHAREFS_GET_SIZE, &minfo);
> > +
> > +c. Map the mshare'd region into the process:
> > +
> > +.. code-block:: c
> > +
> > +	mmap(minfo.start, minfo.size,
> > +		PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> > +
> > +5. To delete the mshare region:
> > +
> > +.. code-block:: c
> > +
> > +		unlink("/sys/fs/mshare/shareme");
> Sphinx reports htmldocs warnings:
>
> Documentation/filesystems/msharefs.rst:25: WARNING: Literal block expected; none found. [docutils]
> Documentation/filesystems/msharefs.rst:38: WARNING: Literal block expected; none found. [docutils]
> Documentation/filesystems/msharefs.rst:82: WARNING: Literal block expected; none found. [docutils]

Thanks. Will fix this.


Anthony


> Thanks.