Re: [RFC,PATCH] dnotify: enhance or replace?

From: Rüdiger Klaehn
Date: Wed Mar 24 2004 - 11:14:33 EST

Alexander Larsson wrote:

I think everyone agrees that dnotify is a POS that needs replacement,
however coming up with a good new API and implementation seems to be
hard (or at least uninteresting to kernel developers).

I want something like this, so I am willing to spend some time implementing it.

I for sure would welcome a sane file change notification API, i.e. one
that doesn't require the use of signals. However, I don't really care
about recursive monitors, and I'm actually unsure if you really want the
DN_EXTENDED functionallity in the kernel. It seems like a great way to
make the kernel use a lot of unswappable memory, unless you limit the
event queues, and if you do that you need to stat all files in userspace
anyway so you can correctly handle queue overflows.

About recursive notification:

Some way to watch for changes on a whole file system is a must. Otherwise there is really no need to replace dnotify. When I start up KDE it watches for 256 different directories in my /home directory. It would probably watch even more directories if it could. With recursive watching it would only need to watch two or three directories recursively.

About the buffer memory usage problem:

I have been testing the current approach for a few days continuously now, and I don't get event buffer overflows even if I watch for all events on "/". Of course the event buffer size should be limited. The current implementation uses 10*4096 bytes, but in most cases starting with a single 4096 byte page should be enough.

Note that the most common events (read and write) are quite small. Currently they are 32 bytes, but it would not be that hard to get them even smaller if nessecary. This is quite good compared to libraries like dazuko that report the complete path for each change.

Extended information about the type of change has been requested by many persons, and it is nessecary for many applications. People have been writing ugly syscall table hacks for this, so they must be really desperate to get this information...

It should be optional though.

I think the most important properties for a good dnotify replacement is:

* Don't use signals or any other global resource that makes it
impossible to use the API in a library thats supposed to be used by all
sorts of applications.


* Get sane semantics. i.e. if a hardlink changes notify a file change in
all directories the file is in. (This is hard though, it needs backlinks
from the inodes to the directories, at least for the directories with a
monitor, something i guess we don't have today.)

This would require large changes, and I think figuring out all aliases to a path might as well be done in userspace. You don't gain much by putting this in the kernel, and it requires a lot of complexity.

* Some way to get an event when the last open fd to the file is closed
after a file change. This means you won't get hundreds of write events
for a single file change. (Of course, you won't catch writes to e.g.
logs which aren't closed, so this has to be optional. But for a desktop,
this is often what you want.)

Should be no problem to add this with the current approach. But it is not that bad if you are getting hundreds of write events for a single file. They are just 32 bytes, so you can just throw them away in the userspace if you are not interested in them.
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