Hi Jan, I wanted to consult with you about preliminary design thoughts for implementing a hierarchical storage manager (HSM) with fanotify. I have been in contact with some developers in the past who were interested in using fanotify to implement HSM (to replace old DMAPI implementation). Basically, FAN_OPEN_PERM + FAN_MARK_FILESYSTEM should be enough to implement a basic HSM, but it is not sufficient for implementing more advanced HSM features. Some of the HSM feature that I would like are: - blocking hook before access to file range and fill that range - blocking hook before lookup of child and optionally create child My thoughts on the UAPI were: - Allow new combination of FAN_CLASS_PRE_CONTENT and FAN_REPORT_FID/DFID_NAME - This combination does not allow any of the existing events in mask - It Allows only new events such as FAN_PRE_ACCESS FAN_PRE_MODIFY and FAN_PRE_LOOKUP - FAN_PRE_ACCESS and FAN_PRE_MODIFY can have optional file range info - All the FAN_PRE_ events are called outside vfs locks and specifically before sb_writers lock as in my fsnotify_pre_modify [1] POC That last part is important because the HSM daemon will need to make modifications to the accessed file/directory before allowing the operation to proceed. Naturally that opens the possibility for new userspace deadlocks. Nothing that is not already possible with permission event, but maybe deadlocks that are more inviting to trip over. I am not sure if we need to do anything about this, but we could make it easier to ignore events from the HSM daemon itself if we want to, to make the userspace implementation easier. Another thing that might be good to do is provide an administrative interface to iterate and abort pending fanotify permission/pre-content events. You must have noticed the overlap between my old persistent change tracking journal and this design. The referenced branch is from that old POC. I do believe that the use cases somewhat overlap and that the same building blocks could be used to implement a persistent change journal in userspace as you suggested back then. Thoughts? Amir. [1] https://github.com/amir73il/linux/commits/fsnotify_pre_modify
