Re: [RFC V2 12/18] famfs_fuse: Plumb the GET_FMAP message/response

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Aug 14, 2025 at 04:36:57PM +0200, Miklos Szeredi wrote:
> On Thu, 14 Aug 2025 at 15:36, Miklos Szeredi <miklos@szeredi.hu> wrote:
> 
> > I'm still hoping some common ground would benefit both interfaces.
> > Just not sure what it should be.
> 
> Something very high level:
> 
>  - allow several map formats: say a plain one with a list of extents
> and a famfs one

Yes, I think that's needed.

>  - allow several types of backing files: say regular and dax dev

"block device", for iomap.

>  - querying maps has a common protocol, format of maps is opaque to this
>  - maps are cached by a common facility

I've written such a cache already. :)

>  - each type of mapping has a decoder module

I don't know that you need much "decoding" -- for famfs, the regular
mappings correspond to FUSE_IOMAP_TYPE_MAPPED.  The one goofy part is
the device cookie in each IO mapping: fuse-iomap maps each block device
you give it to a device cookie, so I guess famfs will have to do the
same.

OTOH you can then have a famfs backed by many persistent memory
devices.

>  - each type of backing file has a module for handling I/O
> 
> Does this make sense?

More or less.

> This doesn't have to be implemented in one go, but for example
> GET_FMAP could be renamed to GET_READ_MAP with an added offset and
> size parameter.  For famfs the offset/size would be set to zero/inf.
> I'd be content with that for now.

I'll try to cough up a RFC v4 next week.

--D

> Thanks,
> Miklos
> 
> >
> > Thanks,
> > Miklos
>