Re: [fuse-devel] OSS Proxy Jack slave

From: Miklos Szeredi
Date: Thu Jun 10 2010 - 07:52:42 EST

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.


> 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?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at