linux-kernel - Re: [PATCH 4/4] FUSE: implement direct mmap

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Message-Id: <E1MOqyM-00026z-D6@pomaz-ex.szeredi.hu>
Date:	Thu, 09 Jul 2009 12:35:26 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	tj@...nel.org
CC:	miklos@...redi.hu, linux-kernel@...r.kernel.org,
	fuse-devel@...ts.sourceforge.net, akpm@...ux-foundation.org,
	npiggin@...e.de
Subject: Re: [PATCH 4/4] FUSE: implement direct mmap

On Thu, 09 Jul 2009, Tejun Heo wrote:
> >> Using non-swappable memory would be fine for most device emulations
> >> but mapping large amount of pages would be problematic.  Hmmm... this
> >> is difficult.  It's a compromise among flexibility, scalability and
> >> code complexity.
> > 
> > What's the difficulty?
> 
> The desire to avoid pinning all the mapped pages.  :-)
> 
> > Allocating pages, giving them an ID and mapping them into various page
> > tables seems simple in contrast to trying to make a tmpfs file be a
> > fuse file at the same time, which the VM is really not prepared for.
> 
> If pinning all the pages are okay, the above should work fine.

If the page allocation is privileged then I think it's okay.

I have only vague ideas how this "direct mmap" could be used for other
things than sound drivers.  For example there's a significant push
towards making network filesystems coherent.  This is currently not
well supported by fuse and adding coherency guarantees to mmap is
especially tricky.  But I'm not sure it really needs a special mmap
interface, that's just one of the options.

Thoughts?

Thanks,
Miklos
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