[RFC PATCH 00/13][V5] overlayfs: Delayed copy up of data

[Vivek Goyal <vgoyal@redhat.com>]

Hi,

Here is the V5 of patches. Following are changes from V4.

- Flipped the meaning of bit. Now OVL_METACOPY is called OVL_UPPERDATA and
  dropped a barrier patch which implemented ordering requirements between
  oi->flags and oi->__upperdentry.

- Now ovl_d_real() does return upper dentry if upper dentry is metacopy only
  dentry.

- Added a fix to ovl_lookup() goto statement.

- Renamed ->metadata_only to ->metacopy

- Replaced BUG_ON() with WARN_ON() in ovl_copy_up_meta_inode_data()

- Added WARN_ON(!OVL_TYPE_ORIGIN(type)) in ovl_getattr()  

- Took care of bunch of code organization comments from Amir.

Original Description
--------------------
In one of the recent converstions, people mentioned that chown/chmod
lead to copy up files as well as data. We could optimize it so that
only metadata is copied up during chown/chmod and data is copied up when
file is opened for WRITE.

This optimization potentially could be useful with containers and user
namespaces. In popular scenario, people end up doing chown() on whole
image directory tree based on container mappings. And this chown copies
up everything, breaking sharing of page cache between containers.

With these patches, only metadat is copied up during chown() and if file
is opened for READ, d_real() returns lower dentry/inode. That way,
different containers can still continue to use page cache. That's the
use case I have in mind.

Basically, I am relying on storing OVL_XATTR_ORIGIN in upper inode
during copy up. I use that information to get to lower inode later and
do data copy up when necessary.

I also store OVL_XATTR_METACOPY in upper inode to mark that only
metadata has been copied up and data copy up still might be required.

Any feedback is helpful.

Thanks
Vivek

Vivek Goyal (13):
  ovl: Put upperdentry if ovl_check_origin() fails
  ovl: Create origin xattr on copy up for all files
  ovl: ovl_check_setxattr() get rid of redundant -EOPNOTSUPP check
  ovl: Provide a mount option metacopy=on/off for metadata copyup
  ovl: During copy up, first copy up metadata and then data
  ovl: Copy up only metadata during copy up where it makes sense
  ovl: A new xattr OVL_XATTR_METACOPY for file on upper
  ovl: Fix ovl_getattr() to get number of blocks from lower
  ovl: Set OVL_UPPERDATA flag during ovl_lookup()
  ovl: Return lower dentry if only metadata copy up took place
  ovl: Introduce read/write barriers around metacopy flag update
  ovl: Do not export metacopy only upper dentry
  ovl: Enable metadata only feature

 fs/overlayfs/Kconfig     |   8 ++++
 fs/overlayfs/copy_up.c   | 116 +++++++++++++++++++++++++++++++++++------------
 fs/overlayfs/inode.c     |  15 +++++-
 fs/overlayfs/namei.c     |  38 +++++++++++++++-
 fs/overlayfs/overlayfs.h |   6 ++-
 fs/overlayfs/ovl_entry.h |   1 +
 fs/overlayfs/super.c     |  67 +++++++++++++++++++++++++--
 fs/overlayfs/util.c      |  54 +++++++++++++++++-----
 8 files changed, 257 insertions(+), 48 deletions(-)

-- 
2.5.5