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

[David Hildenbrand <david@redhat.com>]

On 10.09.25 14:14, Pedro Falcato wrote:
> On Tue, Aug 19, 2025 at 06:03:54PM -0700, Anthony Yznaga wrote:
>> 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.
>> +
> 
> Welp. I really really don't like this API.
> I assume this has been discussed previously, but why do we need a new
> magical pseudofs mounted under some random /sys directory?
> 
> But, ok, assuming we're thinking about something hugetlbfs like, that's not too
> bad, and programs already know how to use it.
> 
>> +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.
>> +
> 
> I don't see why size and alignment needs to be taken into consideration by
> userspace. You can simply establish a mapping and pad it out.
> 
>> +  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);
> 
> Ok, makes sense.
> 
>> +
>> +    b. Establish the size of the region::
>> +
>> +        fallocate(fd, 0, 0, BUF_SIZE);
>> +
>> +      or::
>> +
>> +        ftruncate(fd, BUF_SIZE);
>> +
> 
> Yep.
> 
>> +    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);
> 
> Why?? Do you want to map mappings in msharefs files, that can themselves be
> mapped? Why do we need an ioctl here?
> 
> Really, this feature seems very overengineered. If you want to go the fs route,
> doing a new pseudofs that's just like hugetlb, but without the hugepages, sounds
> like a decent idea. Or enhancing tmpfs to actually support this kind of stuff.
> Or properly doing a syscall that can try to attach the page-table-sharing
> property to random VMAs.
> 
> But I'm wholly opposed to the idea of "mapping a file that itself has more
> mappings, mappings which you establish using a magic filesystem and ioctls".

I don't remember the history (it's been a while) but there was this 
interest of

(a) Sharing page tables for smaller files (not just PUD size etc.)

(b) Supporting also ordinary file systems, not just tmpfs

(c) Having a way to update protection of parts of a mapping and
     immediately have it visible to everyone mapping that area.

In the past, I raised that some VM use cases around virtio-fs would be 
interested in having a "VMA container" that can be updated by the parent 
QEMU process, and what gets mapped in there would be immediately visible 
to the other processes.

I recall that initially I pushed for just generalizing the support for 
shared page tables so it could be used for other file systems. I recall 
problems around that, likely around protection changes etc.

So current mshare really is the idea of having a (let's call it) VMA 
container that can be mapped into processes where all processes will 
observe changes performed by other processes.

I agree that it's complicated, and the semantics are very, very, very weird.

-- 
Cheers

David / dhildenb