Re: Questions about FUSE_NOTIFY_INVAL_ENTRY

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, 20 Aug 2025 at 01:35, Jim Harris <jiharris@nvidia.com> wrote:

> Can we safely depend on the FUSE_NOTIFY_INVAL_ENTRY notifications to trigger FORGET commands for the associated inodes? If not, can we consider adding a new FUSE_NOTIFY_DROP_ENTRY notification that would ask the kernel to release the inode and send a FORGET command when memory pressure or clean-up is needed by the device?

As far as I understand what you want is drop the entry from the cache
*if it is unused*.  Plain FUSE_NOTIFY_INVAL_ENTRY will unhash the
dentry regardless of its refcount, of course FORGET will be sent only
after the reference is released.

FUSE_NOTIFY_INVAL_ENTRY with FUSE_EXPIRE_ONLY will do something like
your desired FUSE_NOTIFY_DROP_ENTRY operation, at least on virtiofs
(fc->delete_stale is on).  I notice there's a fuse_dir_changed() call
regardless of FUSE_EXPIRE_ONLY, which is not appropriate for the drop
case, this can probably be moved inside the !FUSE_EXPIRE_ONLY branch.

The other question is whether something more efficient should be
added. E.g. FUSE_NOTIFY_SHRINK_LOOKUP_CACHE with a num_drop argument
that tells fuse to try to drop this many unused entries?

Thanks,
Miklos