[Linux-cachefs] Re: [PATCH] [Request for inclusion] Filesystem in Userspace

[Miklos Szeredi]

> Check fscache (or cachefs), right now it has the 'problem' that it
> considers all cached pages as purgeable, but David Howell's did express
> some interest in adding the ability to pin pages until the fs has
> written back the data or at least he will probably consider accepting
> patches to that extent.

OK.  It's not clear to me who is the 'owner' of a page.  Is it the
page cache or cachefs?  So what exectly happens when I update a page
in fscache?  Is it copied to an cachefs page and then later written
back to disk, or is it immediately sent to disk?

> Effectively both clean and dirty pages would get backed on disk by
> fscache, this might be advantageous especially if FUSE has to pull the
> data off of some slow medium or from the network. 

That sounds good, though I think it would be better if caching could
be made optional.  Many userspace filesystems are not network
filesystems, and often recreating the data is cheaper than reading
back from a disk cache. 

> If the dirty pages can
> get pinned, then a write would work as follows,
> 
>     writepage
> 	update (and pin) page in the fscache
> 	queue (async) write request to userspace
> 
> Now as far as the kernel is concerned once writepage returns the page is
> clean and can be dropped. Userspace can then use a downcall to pull the
> page out of the fscache and unpin it. I don't know if fscache/cachefs
> has a filesystem like interface which would allow access to the page
> directly with open/seek/read operations without going back through the
> FUSE kernel module.

It sounds to me cleaner if cached data is always accessed through
FUSE, so I don't think such an interface is neccessary, nor that it
would improve performance.

Thanks,
Miklos