[RFC PATCH 0/8] fsdax: introduce FS query interface to support reflink

[Shiyang Ruan]

This patchset is a try to resolve the problem of tracking shared page
for fsdax.

Instead of per-page tracking method, this patchset introduces a query
interface: get_shared_files(), which is implemented by each FS, to
obtain the owners of a shared page.  It returns an owner list of this
shared page.  Then, the memory-failure() iterates the list to be able
to notify each process using files that sharing this page.

The design of the tracking method is as follow:
1. dax_assocaite_entry() associates the owner's info to this page
- For non-reflink case:
  page->mapping,->index stores the file's mapping, offset in file.
    A dax page is not shared by other files. dax_associate_entry() is
    called only once.  So, use page->mapping,->index to store the
    owner's info.
- For reflink case:
  page->mapping,->index stores the block device, offset in device.
    A dax page is shared more than once.  So, dax_assocaite_entry()
    will be called more than once.  We introduce page->zone_device_data
    as reflink counter, to indicate that this page is shared and how
    many owners now is using this page. The page->mapping,->index is
    used to store the block_device of the fs and page offset of this
    device.

2. dax_lock_page() calls query interface to lock each dax entry
- For non-reflink case:
  owner's info is stored in page->mapping,->index.
    So, It is easy to lock its dax entry.
- For reflink case:
  owner's info is obtained by calling get_shared_files(), which is
  implemented by FS.
    The FS context could be found in block_device that stored by
    page->mapping.  Then lock the dax entries of the owners.

In memory-failure(), since the owner list has been obtained in 
dax_lock_page(), just iterate the list and handle the error.  This
patchset didn't handle the memory failure on metadata of FS because
I haven't found a way to distinguish whether this page contains
matadata yet.  Still working on it.

==
I also borrowed and made some changes on Goldwyn's patchsets.
These patches makes up for the lack of CoW mechanism in fsdax.

The rests are dax & reflink support for xfs.

(Rebased on v5.8)
==
Shiyang Ruan (8):
  fs: introduce get_shared_files() for dax&reflink
  fsdax, mm: track files sharing dax page for memory-failure
  fsdax: introduce dax_copy_edges() for COW
  fsdax: copy data before write
  fsdax: replace mmap entry in case of CoW
  fsdax: dedup file range to use a compare function
  fs/xfs: handle CoW for fsdax write() path
  fs/xfs: support dedupe for fsdax

 fs/btrfs/reflink.c     |   3 +-
 fs/dax.c               | 302 +++++++++++++++++++++++++++++++++++------
 fs/ocfs2/file.c        |   2 +-
 fs/read_write.c        |  11 +-
 fs/xfs/xfs_bmap_util.c |   6 +-
 fs/xfs/xfs_file.c      |  10 +-
 fs/xfs/xfs_iomap.c     |   3 +-
 fs/xfs/xfs_iops.c      |  11 +-
 fs/xfs/xfs_reflink.c   |  80 ++++++-----
 fs/xfs/xfs_super.c     |  67 +++++++++
 include/linux/dax.h    |  18 ++-
 include/linux/fs.h     |  11 +-
 include/linux/mm.h     |   8 ++
 mm/memory-failure.c    | 138 ++++++++++++-------
 14 files changed, 529 insertions(+), 141 deletions(-)

-- 
2.27.0