From Yu Kuai (with minor changes by Song Liu):

The background is that currently bitmap is using a global spin_lock,
causing lock contention and huge IO performance degradation for all raid
levels.

However, it's impossible to implement a new lock free bitmap with
current situation that md-bitmap exposes the internal implementation
with lots of exported apis. Hence bitmap_operations is invented, to
describe bitmap core implementation, and a new bitmap can be introduced
with a new bitmap_operations, we only need to switch to the new one
during initialization.

And with this we can build bitmap as kernel module, but that's not
our concern for now.

This version was tested with mdadm tests and lvm2 tests. This set does
not introduce new errors in these tests.

* md-6.12-bitmap: (42 commits)
  md/md-bitmap: make in memory structure internal
  md/md-bitmap: merge md_bitmap_enabled() into bitmap_operations
  md/md-bitmap: merge md_bitmap_wait_behind_writes() into bitmap_operations
  md/md-bitmap: merge md_bitmap_free() into bitmap_operations
  md/md-bitmap: merge md_bitmap_set_pages() into struct bitmap_operations
  md/md-bitmap: merge md_bitmap_copy_from_slot() into struct bitmap_operation.
  md/md-bitmap: merge get_bitmap_from_slot() into bitmap_operations
  md/md-bitmap: merge md_bitmap_resize() into bitmap_operations
  md/md-bitmap: pass in mddev directly for md_bitmap_resize()
  md/md-bitmap: merge md_bitmap_daemon_work() into bitmap_operations
  md/md-bitmap: merge bitmap_unplug() into bitmap_operations
  md/md-bitmap: merge md_bitmap_unplug_async() into md_bitmap_unplug()
  md/md-bitmap: merge md_bitmap_sync_with_cluster() into bitmap_operations
  md/md-bitmap: merge md_bitmap_cond_end_sync() into bitmap_operations
  md/md-bitmap: merge md_bitmap_close_sync() into bitmap_operations
  md/md-bitmap: merge md_bitmap_end_sync() into bitmap_operations
  md/md-bitmap: remove the parameter 'aborted' for md_bitmap_end_sync()
  md/md-bitmap: merge md_bitmap_start_sync() into bitmap_operations
  md/md-bitmap: merge md_bitmap_endwrite() into bitmap_operations
  md/md-bitmap: merge md_bitmap_startwrite() into bitmap_operations
  ...
Signed-off-by: Song Liu <song@kernel.org>