[fuse-devel] dentry expiry instead of invalidation

[Jakob B. <jb...@ce...>]

Dear all,

For the development of cvmfs [1], we are wondering if it is possible to 
let a fuse module expire a certain dentry, so that the next access 
triggers a LOOKUP callback.

The background is this: cvmfs is a read-only network file system.  It is 
not immutable -- every now and then a new file system snapshot becomes 
available remotely and is then served by clients.

We recently started looking into symlink kernel caching.  When a symlink 
is updated to a longer one, the client can temporarily serve a truncated 
version of the new, long symlink.  This is due to the kernel cache still 
using the old entry size.

We know that the following options prevent this behavior:

   - wait for the kernel cache timeout; this is not ideal because it 
prevents clients from swiftly following remote changes (or, with short 
timeouts, makes inefficient use of the kernel cache)

   - Invalidate the dentry with fuse_lowlevel_notify_inval_entry() 
before applying the update.

Invalidating the dentry, however, has at least one undesirable 
side-effect: a mountpoint on the symlink gets irrecoverably destroyed.

We would ideally like to be able to immediately "expire" the dentry, so 
that it remains valid but the next access should trigger a LOOKUP callback.

We may be wrong but it seems that the fuse kernel module's 
fuse_change_entry_timeout() would provide that functionality if it was 
exposed through libfuse..?

Cheers,
Jakob

[1] https://github.com/cvmfs/cvmfs