[Ocfs2-devel] [-mm PATCH] ocfs2: Shared writeable mmap

[Andrew Morton]

Mark Fasheh <mark.fasheh at oracle.com> wrote:
>
> I finally got some time to sit down and implement an OCFS2 patch to make use
> of the ->page_mkwrite() callback added by David Howells' patch (named
> 'add-page_mkwrite-vm_operations-method.patch' in -mm). The patches, and an
> MPI program to test this can be found at:
> 
> http://kernel.org/pub/linux/kernel/people/mfasheh/ocfs2/mmap/
> 
> There's one bug however, which will cause the test program on one of the
> reading nodes to see stale data if it is run several times in a row against
> the same file. I have verified that the same thing works fine on a local
> file system (ext3). I'm not sure where the issue is, but I have a feeling
> I'm doing something bad in ocfs2_data_convert_worker(). Another possibility
> is that we missed a place to put the ->page_mkwrite callback.
> 
> Unfortunately, I have to step away from this patch for a bit as I have some
> higher priority issues to deal with :/ Luckily, it seems to be in a state
> which I think warrants it being pushed out to the public for general review,
> testing, etc. If anyone is interested, I'd also appreciate any advice or
> help regarding the bug -- my VM-foo is very weak :)

Peter Zijlstra told me yesterday:

  There is a problem with the page-mkwrite last posted to lkml.  /me
  checks your tree...  Yeah, that version has a problem:
  http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.17-rc6/2.6.17-rc6-mm2/broken-out/add-page_mkwrite-vm_operations-method.patch

  The thing is that get_user_pages(.write=1, .force=1) can generate COW
  hits on read-only shared mappings, this patch traps those as mkpage_write
  candidates and fails to handle them the old way.

Which I was unaware of and haven't started to think about.  Probably I'll
drop the existing patch and pick up the one he's sending out.  But it's
presumably based on top of all the dirty-page-tracking patches which I also
haven't thought about yet and which need _serious_ thought.

It would be better to get a fix against the existing
add-page_mkwrite-vm_operations-method.patch so at least we can get that
merged up.  But nobody seems to be offering that.