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From: Daniel Phillips <phillips@bonn-fries.net>
To: "Albert D. Cahalan" <acahalan@cs.uml.edu>
Subject: Re: File copy system call proposal
Date: Mon, 10 Dec 2001 16:19:33 +0100
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On December 10, 2001 06:44 am, Albert D. Cahalan wrote:
> Daniel Phillips writes:
> 
> > There's some merit to this idea.  As Peter pointed out,
> > an in-kernel cp isn' needed: mmap+write does the job.
> > The question is, how to avoid the copy_from_user and
> > double caching of data?
> 
> No, mmap+write does not do the job. SMB file servers have
> a remote copy operation. There shouldn't be any need to
> pull data over the network only to push it back again!

Hi Albert,

I don't get it, you're saying that this zero-copy optimization, which happens 
entirely within the vfs, shouldn't be done because smb can't do it over a 
network?

> The user-space copy operation is also highly likely to
> lose stuff that the kernel would know about:
> 
> extended attributes   (IRIX, OS/2, NT)
> forks / extra streams   (MacOS, NT)
> creation time stamp   (Microsoft: not ctime or mtime)
> author   (GNU HURD: person who created the file)
> file type   (MacOS)
> creator app   (MacOS)
> unique ID   (Win2K)
> mandatory access control data   (Trusted Foo)
> non-UNIX permission bits   (every other OS)
> ACLs   (NFSv4, NT, Solaris...)
> translator   (HURD)
> trustees   (NetWare)

I'd think the mmap-based copy would only use the technique on the data 
portion of a file.

Note that I'm not seriously proposing to do this, there are about 1,000 more 
important things.  I'm suggesting the original poster go take a look at the 
issues involved in making it happen.

--
Daniel
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