[CC += Al Viro & Linux, since they also discussed the point about remote filesystems and /proc and /sys here: http://thread.gmane.org/gmane.linux.file-systems/83641/focus=83713 .] On 04/03/2014 05:38 PM, Eric W. Biederman wrote: > "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 Good point. I recently got CCed on that very point, but hadn't added it to the page. I've added it now. Revised text below, after incorporating changes from your comments and those of Jan Kara. Cheers, 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. Inotify reports only events that a user-space program triggers through the filesystem API. As a result, it does not catch remote events that occur on network filesystems. (Applications must fall back to polling the filesystem to catch such events.) Furthermore, various virtual filesystems such as /proc, /sys, and /dev/pts are not monitorable with inotify. 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 As noted above, the IN_MOVED_FROM and IN_MOVED_TO event pair that is generated by rename(2) can be matched up via their shared cookie value. However, the task of matching has some challenges. These two events are usually consecutive in the event stream available when reading from the inotify file descriptor. How‐ ever, this is not guaranteed. If multiple processes are trigger‐ ing events for monitored 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). -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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
