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

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Message-ID: <87k4ulo5w2.fsf@frosties.localdomain>
Date:	Wed, 10 Feb 2010 09:24:13 +0100
From:	Goswin von Brederlow <goswin-v-b@....de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Miklos Szeredi <mszeredi@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	fuse-devel@...ts.sourceforge.net,
	Lars Wendler <polynomial-c@...too.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support

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?

MfG
        Goswin
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