Re: Things I wish I'd known about Inotify

[Jan Kara]

On Fri 04-04-14 09:35:50, Michael Kerrisk (man-pages) wrote:
> On 04/03/2014 10:52 PM, Jan Kara wrote:
> > On Thu 03-04-14 08:34:44, Michael Kerrisk (man-pages) wrote:
> >>    Limitations and caveats
> >>        The inotify API provides no information about the user or process
> >>        that triggered the inotify event.  In  particular,  there  is  no
> >>        easy  way  for a process that is monitoring events via inotify to
> >>        distinguish events that it triggers itself from  those  that  are
> >>        triggered by other processes.
> >>
> >>        The  inotify API identifies affected files by filename.  However,
> >>        by the time an application processes an inotify event, the  fileâ
> >>        name may already have been deleted or renamed.
> >>
> >>        The  inotify  API identifies events via watch descriptors.  It is
> >>        the application's responsibility to cache a mapping  (if  one  is
> >>        needed)  between  watch descriptors and pathnames.  Be aware that
> >>        directory renamings may affect multiple cached pathnames.
> >>
> >>        Inotify monitoring of directories is not  recursive:  to  monitor
> >>        subdirectories under a directory, additional watches must be creâ
> >>        ated.  This can take a significant amount time for  large  direcâ
> >>        tory trees.
> >   And also there's a problem with the limit on the number of watches a user
> > can have.
> 
> What is the problem exactly (given that the limit is configurable)?
  Well, if you want to watch the whole home directory and you have a large
one, you may run into that limit. Sure you can ask sysadmin to raise the
limit but it's a bit anoying.

> >>        If monitoring an entire directory subtree, and a new subdirectory
> >>        is created in that tree or an existing directory is renamed  into
> >>        that  tree,  be aware that by the time you create a watch for the
> >>        new subdirectory, new  files  (and  subdirectories)  may  already
> >>        exist  inside  the subdirectory.  Therefore, you may want to scan
> >>        the contents of the subdirectory  immediately  after  adding  the
> >>        watch (and, if desired, recursively add watches for any subdirecâ
> >>        tories that it contains).
> >>
> >>        Note that the event queue can overflow.  In this case, events are
> >>        lost.   Robust applications should handle the possibility of lost
> >>        events gracefully.  For example, it may be necessary  to  rebuild
> >>        part  or all of the application cache.  (One simple, but possibly
> >>        expensive, approach is to  close  the  inotify  file  descriptor,
> >>        empty  the  cache, create a new inotify file descriptor, and then
> >>        re-create watches and cache entries for the objects to  be  moniâ
> >>        tored.)
> >>
> >>    Dealing with rename() events
> >>        The  IN_MOVED_FROM  and  IN_MOVED_TO events that are generated by
> >>        rename(2) are usually available as consecutive events when  readâ
> >>        ing from the inotify file descriptor.  However, this is not guarâ
> >>        anteed.  If multiple processes are triggering  events  for  moniâ
> >>        tored  objects,  then  (on rare occasions) an arbitrary number of
> >>        other events may appear between the IN_MOVED_FROM and IN_MOVED_TO
> >>        events.
> >>
> >>        Matching  up  the IN_MOVED_FROM and IN_MOVED_TO event pair generâ
> >>        ated by rename(2) is thus inherently racy.  (Don't forget that if
> >>        an  object is renamed outside of a monitored directory, there may
> >>        not even be an IN_MOVED_TO event.)  Heuristic  approaches  (e.g.,
> >>        assume the events are always consecutive) can be used to ensure a
> >>        match in most cases, but will inevitably miss some cases, causing
> >>        the  application  to  perceive  the IN_MOVED_FROM and IN_MOVED_TO
> >>        events as being unrelated.  If watch  descriptors  are  destroyed
> >>        and  re-created as a result, then those watch descriptors will be
> >>        inconsistent with the watch descriptors in  any  pending  events.
> >>        (Re-creating the inotify file descriptor and rebuilding the cache
> >>        may be useful to deal with this scenario.)
> >   Well, but there's 'cookie' value meant exactly for matching up
> > IN_MOVED_FROM and IN_MOVED_TO events. And 'cookie' is guaranteed to be
> > unique at least within the inotify instance (in fact currently it is unique
> > within the whole system but I don't think we want to give that promise).
> 
> Yes, that's already assumed by my discussion above (its described elsewhere
> in the page). But your comment makes me think I should add a few words to
> remind the reader of that fact. I'll do that.
  Yes, that would be good.

> But, the point is that even with the cookie, matching the events is 
> nontrivial, since:
> 
> * There may not even be an IN_MOVED_FROM event
> * There may be an arbitrary number of other events in between the 
>   IN_MOVED_FROM and the IN_MOVED_TO.
> 
> Therefore, one has to use heuristic approaches such as "allow at least
> N millisconds" or "check the next N events" to see if there is an
> IN_MOVED_FROM that matches the IN_MOVED_TO. I can't see any way around
> that being inherently racy. (It's unfortunate that the kernel can't 
> provide a guarantee that the two events are always consecutive, since
> that would simply user space's life considerably.)
  Yeah, it's unpleasant but doing that would be quite costly/complex at the
kernel side. And the race would in the worst case lead to application
thinking there's been file moved outside of watched area & a file moved
somewhere else inside the watched area. So the application will have to
possibly inspect that file. That doesn't seem too bad.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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