mirror of
https://github.com/torvalds/linux.git
synced 2024-11-23 20:51:44 +00:00
a5b2f95d0c
The inotify interface has changed a lot. The user interface was too
old, and the kernel interface was removed by Eric Paris in commit:
2dfc1ca inotify: remove inotify in kernel interface.
Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Cc: Wang Kai <morgan.wang@huawei.com>
Cc: Eric Paris <eparis@parisplace.org>
Cc: Robert Love <robert.w.love@intel.com>
Cc: John McCutchan <john@johnmccutchan.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
80 lines
3.8 KiB
Plaintext
80 lines
3.8 KiB
Plaintext
inotify
|
|
a powerful yet simple file change notification system
|
|
|
|
|
|
|
|
Document started 15 Mar 2005 by Robert Love <rml@novell.com>
|
|
Document updated 4 Jan 2015 by Zhang Zhen <zhenzhang.zhang@huawei.com>
|
|
--Deleted obsoleted interface, just refer to manpages for user interface.
|
|
|
|
(i) Rationale
|
|
|
|
Q: What is the design decision behind not tying the watch to the open fd of
|
|
the watched object?
|
|
|
|
A: Watches are associated with an open inotify device, not an open file.
|
|
This solves the primary problem with dnotify: keeping the file open pins
|
|
the file and thus, worse, pins the mount. Dnotify is therefore infeasible
|
|
for use on a desktop system with removable media as the media cannot be
|
|
unmounted. Watching a file should not require that it be open.
|
|
|
|
Q: What is the design decision behind using an-fd-per-instance as opposed to
|
|
an fd-per-watch?
|
|
|
|
A: An fd-per-watch quickly consumes more file descriptors than are allowed,
|
|
more fd's than are feasible to manage, and more fd's than are optimally
|
|
select()-able. Yes, root can bump the per-process fd limit and yes, users
|
|
can use epoll, but requiring both is a silly and extraneous requirement.
|
|
A watch consumes less memory than an open file, separating the number
|
|
spaces is thus sensible. The current design is what user-space developers
|
|
want: Users initialize inotify, once, and add n watches, requiring but one
|
|
fd and no twiddling with fd limits. Initializing an inotify instance two
|
|
thousand times is silly. If we can implement user-space's preferences
|
|
cleanly--and we can, the idr layer makes stuff like this trivial--then we
|
|
should.
|
|
|
|
There are other good arguments. With a single fd, there is a single
|
|
item to block on, which is mapped to a single queue of events. The single
|
|
fd returns all watch events and also any potential out-of-band data. If
|
|
every fd was a separate watch,
|
|
|
|
- There would be no way to get event ordering. Events on file foo and
|
|
file bar would pop poll() on both fd's, but there would be no way to tell
|
|
which happened first. A single queue trivially gives you ordering. Such
|
|
ordering is crucial to existing applications such as Beagle. Imagine
|
|
"mv a b ; mv b a" events without ordering.
|
|
|
|
- We'd have to maintain n fd's and n internal queues with state,
|
|
versus just one. It is a lot messier in the kernel. A single, linear
|
|
queue is the data structure that makes sense.
|
|
|
|
- User-space developers prefer the current API. The Beagle guys, for
|
|
example, love it. Trust me, I asked. It is not a surprise: Who'd want
|
|
to manage and block on 1000 fd's via select?
|
|
|
|
- No way to get out of band data.
|
|
|
|
- 1024 is still too low. ;-)
|
|
|
|
When you talk about designing a file change notification system that
|
|
scales to 1000s of directories, juggling 1000s of fd's just does not seem
|
|
the right interface. It is too heavy.
|
|
|
|
Additionally, it _is_ possible to more than one instance and
|
|
juggle more than one queue and thus more than one associated fd. There
|
|
need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
|
|
process can easily want more than one queue.
|
|
|
|
Q: Why the system call approach?
|
|
|
|
A: The poor user-space interface is the second biggest problem with dnotify.
|
|
Signals are a terrible, terrible interface for file notification. Or for
|
|
anything, for that matter. The ideal solution, from all perspectives, is a
|
|
file descriptor-based one that allows basic file I/O and poll/select.
|
|
Obtaining the fd and managing the watches could have been done either via a
|
|
device file or a family of new system calls. We decided to implement a
|
|
family of system calls because that is the preferred approach for new kernel
|
|
interfaces. The only real difference was whether we wanted to use open(2)
|
|
and ioctl(2) or a couple of new system calls. System calls beat ioctls.
|
|
|