On Tue, Sep 19, 2023 at 12:01 PM Jan Kara <jack@xxxxxxx> wrote: > So with inotify event, you get back 'wd' and 'name' to identify the object > where the event happened. How is this (for your usecase) different from > getting back 'fsid + handle' and 'name' back from fanotify? In inotify case > you had to somehow track wd -> path linkage, with fanotify you need to > track 'fsid + handle' -> path linkage. The wd is a simple "int" which is the return value of the system call, and it's part of "struct inotify_event". One system call for registering it, one system call fo reading it. >From fanotify, I read a "struct fanotify_event_metadata", and then check variable-length follow-up structs, iterable those follow-up structs, find the one with "info_type==FAN_EVENT_INFO_TYPE_FID", now I have a "fsid" of type "__kernel_fsid_t" (a struct containing two 32-bit integers) and a "file_handle" (a variable-length opaque BLOB). What do I do with these? The answer appears to be: when I registered, I should have obtained the fsid (via statfs()) and the file_handle (via name_to_handle_at()). That's three extra system calls. One statfs(), and twice name_to_handle_at(), because the first one is needed to get the length of the buffer I need to allocate for the file_handle (and hope my filesystem supports file_handles, because apparently that's an optional feature). Just look at the name_to_handle_at() manpage for some horrors of its complexity. Imagine how much more complex the data structure for looking up the modified file is: inotify has an int as the lookup key, and fanotify has two integers plus a variable-length BLOB. > But if you want to monitor multiple filesystems I can monitor multiple filesystems with inotify. > or if you have priviledged process that can open by handle Getting an already-opened file descriptor, or just the file_handle, is certainly an interesting fanotify feature. But that could have easily been added to inotify with a new "mask" flag for the inotify_add_watch() function. > or a standard filesystem where handles are actually persistent, then there are benefits. Same here: that could have been an (optional) inotify feature, instead of making the whole complexity mandatory for everybody. Max
