Re: File copy system call proposal

From: Daniel Phillips (
Date: Mon Dec 10 2001 - 10:19:33 EST

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

> 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.

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This archive was generated by hypermail 2b29 : Sat Dec 15 2001 - 21:00:17 EST