Re: [PATCH] cowlinks v2

[Jamie Lokier <jamie@shareable.org>]

Eric W. Biederman wrote:
> > 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.
> 
> Exactly so they would not share the same pages in RAM.

That is one way to implement it.

> > Btw, I'm not suggesting sharing page cache entries.
> 
> It sounded like you assumed sharing of page cache entries above.  
> How do you get to step 2 if the cow copies don't share the same page
> cache entries?

Ah.  A misunderstanding on my part.

I mean not sharing page cache entries between different
address_spaces, but sharing between different cowlinks which use the
same underlying address_space.

I had in mind that since each cowlink is a separate inode, but both
inodes point to a shared data structure in the filesystem, they would
map pages out of a shared address_space representing that data
structure.  You've pointed out that it isn't necessary to do that, and
it's probably simpler not to.

Now I see your point.  Page sharing could be avoided completely, if
when mapping a cowlink the page was _copied_ from the shared
address_space to the cowlink's own address_space.  Copying also solves
the mlock() problem.  (A shared address_space is still required, because
you may cowlink a file which has dirty pages in RAM).

Copying raises a different problem: what to do when a non-cowlink file
is mapped (PROT_READ), and then it's cowlinked while the mapping is in
place.  The non-cowlink inode gets converted to a cowlink inode.  The
pages are hashed in the original address_space, and you now have a
mapping of a cowlink file where the mapped pages are _not copies_ of
pages in the shared address_space.

-- Jamie