To: Tejun Heo <tj@kernel.org>
CC: miklos@szeredi.hu, mszeredi@suse.cz, linux-kernel@vger.kernel.org,
       fuse-devel@lists.sourceforge.net, polynomial-c@gentoo.org,
       akpm@linux-foundation.org
In-reply-to: <4B74065D.2000707@kernel.org> (message from Tejun Heo on Thu, 11
	Feb 2010 22:30:05 +0900)
Subject: Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support
References: <4B70FBE4.7050700@kernel.org> <E1NerZ6-0007N6-UY@pomaz-ex.szeredi.hu> <4B7296DF.207@kernel.org> <E1NfAl6-0000o4-8Z@pomaz-ex.szeredi.hu> <4B729F07.8020704@kernel.org> <E1NfBTc-0000vk-59@pomaz-ex.szeredi.hu> <4B72A802.6040009@kernel.org> <E1NfE50-0001EK-Ao@pomaz-ex.szeredi.hu> <4B7344A4.1030607@kernel.org> <E1NfVOi-0002l9-5l@pomaz-ex.szeredi.hu> <4B73EE68.4070004@kernel.org> <E1NfYFm-00034C-Pd@pomaz-ex.szeredi.hu> <4B73FE96.2080707@kernel.org> <E1NfYfb-00039p-Uh@pomaz-ex.szeredi.hu> <4B74065D.2000707@kernel.org>
Message-Id: <E1NfZHO-0003FY-8E@pomaz-ex.szeredi.hu>
From: Miklos Szeredi <miklos@szeredi.hu>
Date: Thu, 11 Feb 2010 14:40:26 +0100
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1353
Lines: 41

On Thu, 11 Feb 2010, Tejun Heo wrote:
> > So there are different offsets:
> > 
> >  a) vma->vm_pgoff (which may mean anything, but usually means b)
> 
> Yeap, vma->vm_pgoff can be any value and doesn't really matter.  The
> only visible difference would be the /proc listing, right?  Setting
> this to the requested offset is trivial.

You mean leaving it at the requested offset?  Yes, that's the most
trivial thing to do.  Very few drivers change vm_pgoff:

  git grep "vm_pgoff *=[^=]"

> >  b) the offset at which the pages of the mapping are located
> >  c) the offset at which the server side mmap is located
> 
> There are three offsets.
> 
> a) the offset a client requested
> 
> b) the offset into dmmap AS, a client mmap region is mapped to.  This
>    could be different from a) by multiple of SHMLBA / PAGE_SIZE.

No, it could be different from a) by an arbitrary value.

> 
> c) the offset into dmmap AS, a server mmap region is mapped to, where
>    collection of these mmaps define the dmmap AS.
> 
> The offsets used in b) and c) are the same offsets.

Why are they the same?

Thanks,
Miklos
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