linux-kernel - Re: [GIT PULL] kdbus for 4.1-rc1

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Date:	Wed, 15 Apr 2015 23:48:54 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Richard Weinberger <richard.weinberger@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>, Tom Gundersen <teg@...m.no>,
	Jiri Kosina <jkosina@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	David Herrmann <dh.herrmann@...il.com>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1

On Wed, Apr 15, 2015 at 03:28:58PM -0700, Andy Lutomirski wrote:
> On Wed, Apr 15, 2015 at 3:18 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > On Wed, Apr 15, 2015 at 03:11:17PM -0700, Andy Lutomirski wrote:
> >
> >> This is functionally identical to passing AF_UNIX socket fds over
> >> SCM_RIGHTS, but I want something much lighter weight.
> >
> > Most of the weight in SCM_RIGHTS comes from the fact that you can
> > pass AF_UNIX sockets over it, which requires a garbage collector.
> > Exclude that and suddenly it becomes very cheap...
> 
> I should have been more specific.  I don't mean the performance of
> SCM_RIGHTS itself; I mean the memory overhead of keeping tons of fds
> around, each with their socket data structures and buffers.
> 
> I think that dbus could be quite efficiently implemented with a
> userspace daemon that just introduces peers to each other, but the fd
> explosion could be rather bad for some use cases.
> 
> I'll be the first to admit that I don't have a clean API in mind.
> There was a lightweight fd proposal way back when, but it never went
> anywhere, and it might not be suitable anyway.

Wait, are you talking about the overhead of descriptors used for capability
tokens (essentially zero - one system-wide struct file per capability +
one pointer in descriptor table of anyone who holds it + two bits in
bitmaps in the sam descriptor tables) or about the overhead of descriptors
used to send/receive those over?  The latter don't have to be sockets
at all - they could bloody well be files on some ipcfs, or character device,
or FIFOs, etc.
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