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

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Message-ID: <4A55286A.6040202@kernel.org>
Date:	Thu, 09 Jul 2009 08:14:50 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	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

Hello, Miklos.

Miklos Szeredi wrote:
> 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?

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.

Thanks.

-- 
tejun
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