Re: [PATCH] cowlinks v2

[Jamie Lokier]

Eric W. Biederman wrote:
> > Here's a tricky situation:
> > 
> >    1. A file is cowlinked.  Then each cowlink is mmap()'d, one per process.
> > 
> >    2. At this point both mappings share the same pages in RAM.
> 
> Why they have different inodes?

Did you miss the last 20 or so messages in this thread?

We'd like cowlinks that are an invisible filesystem optimisation.
That means you "copy" a file and it behaves the same as if you copy a file.

> >    3. Then one of the cowlinks is written to...
> 
> I would not worry about sharing page cache entries unless this becomes
> a common case.  If you want to avoid the hit of rereading the file when
> you have a cow copy it should be simple enough to walk through the list
> of cow copies and see if anyone else has it open.

It is not a question of performance, it's correctness.  Either you
have cowlinks that behave like copied files, or you accept that when
cowlinked files are mmapped and written to, they don't behave like
regular files (not even the original file prior to cowlinking does).

Btw, I'm not suggesting sharing page cache entries.

-- Jamie
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