"Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes: > (To: == [the set of people I believe know a lot about inotify]) > > Hello all, > > Lately, I've been studying the inotify API fairly thoroughly and > realized that there's a very big gap between knowing what the system > calls do versus using them to reliably and efficiently monitor the > state of a set of filesystem objects. > > With that in mind, I've drafted some substantial additions to the > inotify(7) man page. I would be very happy if folk on the "To:" list > could comment on the text below, since I believe you all have a lot of > practical experience with Inotify. (Of course, I also welcome comments > from anyone else.) In particular, I would like comments on the > accuracy of the various technical points (especially those relating to > matching up related IN_MOVED_FROM and IN_MOVED_TO events), as well as > pointers on any other pitfalls that the programmers should be wary of > that should be added to the page. Other pitfalls. Inotify only report events that a user space program triggers through the filesystem API. Which means inotify is limited for remote filesystems, and filesystems like proc and sys have no monitorable events. Eric > Thanks, > > Michael > > 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 file‐ > 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 cre‐ > ated. This can take a significant amount time for large direc‐ > tory trees. > > 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 subdirec‐ > 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 moni‐ > 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 read‐ > ing from the inotify file descriptor. However, this is not guar‐ > anteed. If multiple processes are triggering events for moni‐ > 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 gener‐ > 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.) > > Applications should also allow for the possibility that the > IN_MOVED_FROM event was the last event that could fit in the buf‐ > fer returned by the current call to read(2), and the accompanying > IN_MOVED_TO event might be fetched only on the next read(2). -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
