Messages in this thread | | | From | Miklos Szeredi <> | Date | Fri, 17 May 2024 11:55:38 +0200 | Subject | Re: [RFC PATCH 00/20] Introduce the famfs shared-memory file system |
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On Thu, 29 Feb 2024 at 07:52, Amir Goldstein <amir73il@gmail.com> wrote:
> I'm not virtiofs expert, but I don't think that you are wrong about this. > IIUC, virtiofsd could map arbitrary memory region to any fuse file mmaped > by virtiofs client. > > So what are the gaps between virtiofs and famfs that justify a new filesystem > driver and new userspace API?
Let me try to fill in some gaps. I've looked at the famfs driver (even tried to set it up in a VM, but got stuck with the EFI stuff).
- famfs has an extent list per file that indicates how each page within the file should be mapped onto the dax device, IOW it has the following mapping:
[famfs file, offset] -> [offset, length]
- fuse can currently map a fuse file onto a backing file:
[fuse file] -> [backing file]
The interface for the latter is
backing_id = ioctl(dev_fuse_fd, FUSE_DEV_IOC_BACKING_OPEN, backing_map); .. fuse_open_out.flags |= FOPEN_PASSTHROUGH; fuse_open_out.backing_id = backing_id;
This looks suitable for doing the famfs file - > dax device mapping as well. I wouldn't extend the ioctl with extent information, since famfs can just use FUSE_DEV_IOC_BACKING_OPEN once to register the dax device. The flags field could be used to tell the kernel to treat this fd as a dax device instead of a a regular file.
Letter, when the file is opened the extent list could be sent in the open reply together with the backing id. The fuse_ext_header mechanism seems suitable for this.
And I think that's it as far as API's are concerned.
Note: this is already more generic than the current famfs prototype, since multiple dax devices could be used as backing for famfs files, with the constraint that a single file can only map data from a single dax device.
As for implementing dax passthrough, I think that needs a separate source file, the one used by virtiofs (fs/fuse/dax.c) does not appear to have many commonalities with this one. That could be renamed to virtiofs_dax.c as it's pretty much virtiofs specific, AFAICT.
Comments? Am I missing something significant?
Thanks, Miklos
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