On Thu, Jun 18, 2015 at 10:25:08AM +0200, Beata Michalska wrote: > On 06/18/2015 01:06 AM, Dave Chinner wrote: > > On Tue, Jun 16, 2015 at 03:09:30PM +0200, Beata Michalska wrote: > >> Introduce configurable generic interface for file > >> system-wide event notifications, to provide file > >> systems with a common way of reporting any potential > >> issues as they emerge. > >> > >> The notifications are to be issued through generic > >> netlink interface by newly introduced multicast group. > >> > >> Threshold notifications have been included, allowing > >> triggering an event whenever the amount of free space drops > >> below a certain level - or levels to be more precise as two > >> of them are being supported: the lower and the upper range. > >> The notifications work both ways: once the threshold level > >> has been reached, an event shall be generated whenever > >> the number of available blocks goes up again re-activating > >> the threshold. > >> > >> The interface has been exposed through a vfs. Once mounted, > >> it serves as an entry point for the set-up where one can > >> register for particular file system events. > >> > >> Signed-off-by: Beata Michalska <b.michalska@xxxxxxxxxxx> > > > > This has massive scalability problems: .... > > Have you noticed that the filesystems have percpu counters for > > tracking global space usage? There's good reason for that - taking a > > spinlock in such a hot accounting path causes severe contention. .... > > Then puts the entire netlink send path inside this spinlock, which > > includes memory allocation and all sorts of non-filesystem code > > paths. And it may be inside critical filesystem locks as well.... > > > > Apart from the serialisation problem of the locking, adding > > memory allocation and the network send path to filesystem code > > that is effectively considered "innermost" filesystem code is going > > to have all sorts of problems for various filesystems. In the XFS > > case, we simply cannot execute this sort of function in the places > > where we update global space accounting. > > > > As it is, I think the basic concept of separate tracking of free > > space if fundamentally flawed. What I think needs to be done is that > > filesystems need access to the thresholds for events, and then the > > filesystems call fs_event_send_thresh() themselves from appropriate > > contexts (ie. without compromising locking, scalability, memory > > allocation recursion constraints, etc). > > > > e.g. instead of tracking every change in free space, a filesystem > > might execute this once every few seconds from a workqueue: > > > > event = fs_event_need_space_warning(sb, <fs_free_space>) > > if (event) > > fs_event_send_thresh(sb, event); > > > > User still gets warnings about space usage, but there's no runtime > > overhead or problems with lock/memory allocation contexts, etc. > > Having fs to keep a firm hand on thresholds limits would indeed be > far more sane approach though that would require each fs to > add support for that and handle most of it on their own. Avoiding >> this was the main rationale behind this rfc. > If fs people agree to that, I'll be more than willing to drop this > in favour of the per-fs tracking solution. > Personally, I hope they will. I was hoping that you'd think a little more about my suggestion and work out how to do background threshold event detection generically. I kind of left it as "an exercise for the reader" because it seems obvious to me. Hint: ->statfs allows you to get the total, free and used space from filesystems in a generic manner. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
