linux-kernel - Re: [fuse-devel] OSS Proxy Jack slave

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Message-Id: <E1OMgIz-0005Wb-2a@pomaz-ex.szeredi.hu>
Date:	Thu, 10 Jun 2010 13:52:17 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Tejun Heo <tj@...nel.org>
CC:	miklos@...redi.hu, hmajxxlh@...bac.com,
	fuse-devel@...ts.sourceforge.net, mszeredi@...e.cz,
	linux-kernel@...r.kernel.org
Subject: Re: [fuse-devel] OSS Proxy Jack slave

On Thu, 10 Jun 2010, Tejun Heo wrote:
> > First, I think server side mmap might be nice to have but not strictly
> > necessary.  I looked at osspd and it just memcopies in and out of the
> > mmaped ring buffer.  Replacing those memcopies with explicit syscalls
> > to get and put the data should work fine.  I doubt that the latency or
> > CPU overhead introduced by the syscalls would actually matter in
> > practice.
> 
> The latency and CPU overhead perse aren't problematic and for osspd,
> copying in and out should be just fine as all update events are
> clearly denoted.

Good.

> How does the kernel know when to issue store or retrieve?

No, it's userspace that initiates store and retrieve.

> > Next thing is how to deal with multiple buffers for each char device.
> > For the above to continue to work we need to make sure there's a
> > separate nodeid associated with each buffer.  The most general thing
> > would be if MMAP reply contained a nodeid which identified the buffer.
> > 
> > Do you see any issues with the above?
> 
> It relates to the previous question but mmap can also be used without
> all updates being notified by some kind of event where the server is
> expected to watch the mmapped area and react which is okay if server
> can share the mapped page but if it has to poll by copying data out of
> kernel buffer each time, it can get prohibitively expensive unless it
> can ask kernel "what changes since when?" which would be pretty nasty
> to implement.

If necessary we could export page protection and page fault interfaces
to userspace, which would allow it to watch for changes.

But that's not needed for OSSP, right?

Thanks,
Miklos
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