lkml.org 
[lkml]   [2004]   [Apr]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] cowlinks v2
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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 14:02    [W:0.142 / U:0.812 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site