[RFC][PATCH] inotify 0.9.4

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To: linux-kernel@vger.kernel.org, nautilus-list@gnome.org, gamin-list@gnome.org, rml@ximian.com, viro@parcelfarce.linux.theplanet.co.uk, akpm@osdl.org, iggy@gentoo.org
Date:	Wed, 22 Sep 2004 20:15:22 -0400

Hello,

Here is release 0.9.4 of inotify. Attached is a patch to 2.6.8.1

--New in this version--
-locking audit (rml)
-fix for more than 1024 watches (rml)
-fix struct padding (rml)
-don't hard code filename length (rml)
-check kmalloc return (rml)
-code style clean ups (rml)
-don't queue duplicate events (me)
-send user space a Q_OVERFLOW event when the kernel event queue
overflows (me)
-misc changes (me)

I would appreciate design and code review and of course testing.

John McCutchan

Release notes:

--Why Not dnotify and Why inotify (By Robert Love)--

Everyone seems quick to deride the blunder known as "dnotify" and
applaud a
replacement, any replacement, man anything but that current mess, but in
the
name of fairness I present my treatise on why dnotify is what one might
call
not good:

* dnotify requires the opening of one fd per each directory that you
intend to
  watch.
        o The file descriptor pins the directory, disallowing the backing
          device to be unmounted, which absolutely wrecks havoc with removable
          media.
        o Watching many directories results in many open file descriptors,
          possibly hitting a per-process fd limit.
* dnotify is directory-based. You only learn about changes to
directories.
  Sure, a change to a file in a directory affects the directory, but you
are
  then forced to keep a cache of stat structures around to compare
things in
  order to find out which file.
* dnotify's interface to user-space is awful.
        o dnotify uses signals to communicate with user-space.
        o Specifically, dnotify uses SIGIO.
        o But then you can pick a different signal! So by "signals," I really
          meant you need to use real-time signals if you want to queue the
          events.
* dnotify basically ignores any problems that would arise in the VFS
from hard
  links.
* Rumor is that the "d" in "dnotify" does not stand for "directory" but
for
  "suck."

A suitable replacement is "inotify." And now, my tract on what inotify
brings
to the table:

* inotify's interface is a device node, not SIGIO.
        o You open only a single fd, to the device node. No more pinning
          directories or opening a million file descriptors.
        o Usage is nice: open the device, issue simple commands via ioctl(),
          and then block on the device. It returns events when, well, there are
          events to be returned.
        o You can select() on the device node and so it integrates with main
          loops like coffee mixed with vanilla milkshake.
* inotify has an event that says "the filesystem that the item you were
  watching is on was unmounted" (this is particularly cool).
* inotify can watch directories or files.
* The "i" in inotify does not stand for "suck" but for "inode" -- the
logical
  choice since inotify is inode-based.

--COMPLEXITY--

I have been asked what the complexity of inotify is. Inotify has
2 path codes where complexity could be an issue:

Adding a watcher to a device
        This code has to check if the inode is already being watched
        by the device, this is O(1) since the maximum number of
        devices is limited to 8.

Removing a watch from a device
        This code has to do a search of all watches on the device to
        find the watch descriptor that is being asked to remove.
        This involves a linear search, but should not really be an issue
        because it is limited to 8192 entries. If this does turn in to
        a concern, I would replace the list of watches on the device
        with a sorted binary tree, so that the search could be done
        very quickly.

The calls to inotify from the VFS code has a complexity of O(1) so
inotify does not affect the speed of VFS operations.

--MEMORY USAGE--

The inotify data structures are light weight:

inotify watch is 40 bytes
inotify device is 68 bytes
inotify event is 272 bytes

So assuming a device has 8192 watches, the structures are only going
to consume 320KB of memory. With a maximum number of 8 devices allowed
to exist at a time, this is still only 2.5 MB

Each device can also have 256 events queued at a time, which sums to
68KB per device. And only .5 MB if all devices are opened and have
a full event queue.

So approximately 3 MB of memory are used in the rare case of
everything open and full.

Each inotify watch pins the inode of a directory/file in memory,
the size of an inode is different per file system but lets assume
that it is 512 byes.

So assuming the maximum number of global watches are active, this would
pin down 32 MB of inodes in the inode cache. Again not a problem
on a modern system.

On smaller systems, the maximum watches / events could be lowered
to provide a smaller foot print.

Keep in mind that this is an absolute worst case memory analysis.
In reality it will most likely cost approximately 5MB.

--HOWTO USE--
Inotify is a character device that when opened offers 2 IOCTL's.
(It actually has 4 but the other 2 are used for debugging)

INOTIFY_WATCH:
        Which takes a path and event mask and returns a unique
        (to the instance of the driver) integer (wd [watch descriptor]
        from here on) that is a 1:1 mapping to the path passed.
        What happens is inotify gets the inode (and ref's the inode)
        for the path and adds a inotify_watcher structure to the inodes
        list of watchers. If this instance of the driver is already
        watching the path, the event mask will be updated and
        the original wd will be returned.

INOTIFY_IGNORE:
        Which takes an integer (that you got from INOTIFY_WATCH)
        representing a wd that you are not interested in watching
        anymore. This will:

        send an IGNORE event to the device
        remove the inotify_watcher structure from the device and
        from the inode and unref the inode.
        
After you are watching 1 or more paths, you can read from the fd
and get events. The events are struct inotify_event. If you are
watching a directory and something happens to a file in the directory
the event will contain the filename (just the filename not the full
path).

--KERNEL CHANGES--
inotify char device driver.

Adding calls to inotify_inode_queue_event and
inotify_dentry_parent_queue_event from VFS operations.
Dnotify has the same function calls. The complexity of the VFS
operations is not affected because inotify_*_queue_event is O(1).

Adding a call to inotify_super_block_umount from
generic_shutdown_superblock

inotify_super_block_umount consists of this:
find all of the inodes that are on the super block being shut down,
sends each watcher on each inode the UNMOUNT and IGNORED event
removes the watcher structures from each instance of the device driver
and each inode.
unref's the inode.

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