Re: [PATCH v6 2/2] fuse: add new function to invalidate cache for all inodes

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 18, 2025 at 03:26:24PM +0100, Miklos Szeredi wrote:
> On Tue, 18 Feb 2025 at 12:51, Luis Henriques <luis@igalia.com> wrote:
> >
> > On Tue, Feb 18 2025, Miklos Szeredi wrote:
> >
> > > On Tue, 18 Feb 2025 at 11:04, Luis Henriques <luis@igalia.com> wrote:
> > >
> > >> The problem I'm trying to solve is that, if a filesystem wants to ask the
> > >> kernel to get rid of all inodes, it has to request the kernel to forget
> > >> each one, individually.  The specific filesystem I'm looking at is CVMFS,
> > >> which is a read-only filesystem that needs to be able to update the full
> > >> set of filesystem objects when a new generation snapshot becomes
> > >> available.
> > >
> > > Yeah, we talked about this use case.  As I remember there was a
> > > proposal to set an epoch, marking all objects for "revalidate needed",
> > > which I think is a better solution to the CVMFS problem, than just
> > > getting rid of unused objects.
> >
> > OK, so I think I'm missing some context here.  And, obviously, I also miss
> > some more knowledge on the filesystem itself.  But, if I understand it
> > correctly, the concept of 'inode' in CVMFS is very loose: when a new
> > snapshot generation is available (you mentioned 'epoch', which is, I
> > guess, the same thing) the inodes are all renewed -- the inode numbers
> > aren't kept between generations/epochs.
> >
> > Do you have any links for such discussions, or any details on how this
> > proposal is being implemented?  This would probably be done mostly in
> > user-space I guess, but it would still need a way to get rid of the unused
> > inodes from old snapshots, right?  (inodes from old snapshots still in use
> > would obvious be kept aroud).
> 
> I don't have links.  Adding Valentin Volkl and Laura Promberger to the
> Cc list, maybe they can help with clarification.
> 
> As far as I understand it would work by incrementing fc->epoch on
> FUSE_INVALIDATE_ALL. When an object is looked up/created the current
> epoch is copied to e.g. dentry->d_time.  fuse_dentry_revalidate() then
> compares d_time with fc->epoch and forces an invalidate on mismatch.

That was what I was going to suggest before I got this far in the
thread. i.e. the fs itself keeps a current snapshot ID, and it keeps
track of what snapshot ID each inode was instantiated for. On access
to the inode it checks the IDs and if they don't match, the inode
needs revalidation and then the fs can take appropriate action.

This also takes care of the active inode reference problem that
invalidation has, in that it can revoke access to inodes that are
not longer valid as soon as they are accessed by userspace.

It is also entirely internal to the filesystem (as I suggested such
functionality should be earlier in the thread) and does not require
walking the VFS to find all cached inodes...

-Dave.
-- 
Dave Chinner
david@fromorbit.com