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

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Message-Id: <E1NfZHO-0003FY-8E@pomaz-ex.szeredi.hu>
Date:	Thu, 11 Feb 2010 14:40:26 +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 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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