Re: [PATCH] cowlinks v2

Davide Libenzi
2004-03-22T05:11:51Z

On 21 Mar 2004, Eric W. Biederman wrote:

> Davide Libenzi <davidel@xmailserver.org> 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