Also, thank you both for the extensive review and ideas during the
development of this series.  It was really appreciated!

Thank you for your appreciated effort!
It was a wild journey through some interesting experiments, but
you survived it well ;-)

Would you be interested in pursuing FAN_WB_ERROR after a due rest
and after all the dust on FAN_FS_ERROR has settled?

FAN_WB_ERROR can use the same info record and same internal
error event struct as FAN_FS_ERROR.

A call to fsnotify_wb_error(sb, inode, err) can be placed inside
mapping_set_error() and maybe for other sporadic callers of errseq_set().

For wb error, we can consider storing a snapshot of errseq of the sb/inode
in the sb/inode mark and compute error_count from the errseq diff
instead of counting it when merging events. This will keep a more accurate
report even when error reports are dropped due to allocation failure or event
queue overflow.

I have a feeling that the Postgres project would find this
functionality useful (?).

Thanks,
Amir.