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

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Message-Id: <E1MNmZM-0001ZB-GR@pomaz-ex.szeredi.hu>
Date:	Mon, 06 Jul 2009 13:41:12 +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 Sat, 04 Jul 2009, Tejun Heo wrote:
> > Afaics sound drivers now map the dma memory with remap_pfn_range().
> > Similary we could allocate a chunk of non-swapabble kernel memory on
> > request from the userspace server and map its pages using this trick
> > to both the server's and the client's address space.
> > 
> > This is still sort of OSSP specific, I don't see clearly how it could
> > be made more generic.
> 
> 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?

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.

Thanks,
Miklos
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