linux-kernel - Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support

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Message-Id: <E1Nf87U-0000QD-7V@pomaz-ex.szeredi.hu>
Date:	Wed, 10 Feb 2010 09:40:24 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Goswin von Brederlow <goswin-v-b@....de>
CC:	tj@...nel.org, fuse-devel@...ts.sourceforge.net,
	polynomial-c@...too.org, akpm@...ux-foundation.org,
	mszeredi@...e.cz, linux-kernel@...r.kernel.org
Subject: Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support

On Wed, 10 Feb 2010, Goswin von Brederlow wrote:
> Tejun Heo <tj@...nel.org> writes:
> 
> > Implement FUSE direct mmap support.  The server can redirect client
> > mmap requests to any SHMLBA aligned offset in the custom address space
> > attached to the fuse channel.  The address space is managed by the
> > server using mmap/munmap(2).  The SHMLBA alignment requirement is
> > necessary to avoid cache aliasing issues on archs with virtually
> > indexed caches as FUSE direct mmaps are basically shared memory
> > between clients and the server.
> >
> > The direct mmap address space is backed by pinned kernel pages which
> > are allocated on the first fault either from a client or the server.
> > If used carelessly, this can easily waste and drain memory.
> > Currently, a server must have CAP_SYS_ADMIN to manage dmmap regions by
> > mmapping and munmapping the channel fd.
> 
> Does that mean that for example in unionfs-fuse when a user wants to
> mmap a file I can just mmap the actual underlying file from the real
> filesystem and any read/write access would then shortcut fuse and go
> directly to the real file?

No.  It is the /dev/fuse file descriptor that is being mapped to give
access to the "direct mmaped" memory.

Thanks,
Miklos
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