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

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Message-Id: <E1NfBTc-0000vk-59@pomaz-ex.szeredi.hu>
Date:	Wed, 10 Feb 2010 13:15:28 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Tejun Heo <tj@...nel.org>
CC:	miklos@...redi.hu, mszeredi@...e.cz, linux-kernel@...r.kernel.org,
	fuse-devel@...ts.sourceforge.net, polynomial-c@...too.org,
	akpm@...ux-foundation.org
Subject: Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support

On Wed, 10 Feb 2010, Tejun Heo wrote:
> > Okay, lets be a little clearer.  There are client side maps and server
> > side maps.  Client side maps are naturally aligned (same offset ->
> > same page).
> 
> Same offset -> same page doesn't hold.

Right, I really meant same page -> same offset.  If the same offset is
mapped to multiple pages: no problem.  If the same page is mapped to
multiple offsets, then obviously it's not going to work properly.

> Can you please elaborate how you think the thing can work without
> referencing the proposed implementation?  Let's find out where the
> misundertanding is.

Thinking about it I'm not really sure...

Maybe the problem is that the propsed solution allows too much
freedom.  Normally there's a 1:1 relationship between pages and
offsets.  But we want to break that for CUSE, because two different
mappings of a char dev might point to completely different pages,
right?

When does that happen?  Can it happen that two mappings of the same
file descriptor will have different backing pages?

Thanks,
Miklos
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