Re: [PATCH] cowlinks v2

[Davide Libenzi]

On 21 Mar 2004, Eric W. Biederman wrote:

> Davide Libenzi <davidel@xxxxxxxxxxxxxxx> writes:
> 
> > > Actually there is...  You don't do the copy until an actual write occurs.
> > > Some files are opened read/write when there is simply the chance they might
> > > be written to so delaying the copy is generally a win.
> > 
> > What about open+mmap?
> 
> The case is nothing really different from having a hole in your file.
> 
> There are two pieces to implementing this.  First you create separate
> page cache  entries for the cow file and it's original, so the
> laziness of mmapped file writes will not bite you..  Second you make
> aops -> writepage trigger the actual copy of the file, and have it
> return -ENOSPC if you can't do the copy.

There has been a misunderstanding. I thought you were talking about a 
userspace solution ala fl-cow. Of course if you are inside the kernel you 
can catch both explicit writes and page syncs.



- Davide


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