[PATCH v4 0/5] mm/hotplug: Only use subsection map for VMEMMAP

[Baoquan He]

Memory sub-section hotplug was added to fix the issue that nvdimm could
be mapped at non-section aligned starting address. A subsection map is
added into struct mem_section_usage to implement it.

However, config ZONE_DEVICE depends on SPARSEMEM_VMEMMAP. It means
subsection map only makes sense when SPARSEMEM_VMEMMAP enabled. For the
classic sparse, subsection map is meaningless and confusing.

About the classic sparse which doesn't support subsection hotplug, Dan
said it's more because the effort and maintenance burden outweighs the
benefit. Besides, the current 64 bit ARCHes all enable
SPARSEMEM_VMEMMAP_ENABLE by default.

This patchset is rebased on the old patch 1 of v3 and an apended fix
which doesn't initialize the local variable 'empty'.

The old patch 7 in v3 will be taken out to post alone, since it's a
clean up patch, not related to this subsection map handling.

Changelog
v3->v4:
  No big change, mainly address concerns from David.

v2->v3:
  David spotted a code bug in the old patch 1, the old local variable
  subsection_map is invalid once ms->usage is resetting. Add a local
  variable 'empty' to cache if subsection_map is empty or not.

  Remove the kernel-doc comments for the newly added functions
  fill_subsection_map() and clear_subsection_map(). Michal and David
  suggested this.

  Add a new static function is_subsection_map_empty() to check if the
  handled section map is empty, but not return the value from
  clear_subsection_map(). David suggested this.

  Add document about only VMEMMAP supporting sub-section hotplug, and
  check_pfn_span() gating the alignment and size. Michal help rephrase
  the words.

v1->v2:
  Move the hot remove fixing patch to the front so that people can
  back port it to easier. Suggested by David.

  Split the old patch which invalidate the sub-section map in
  !VMEMMAP case into two patches, patch 4/7, and patch 6/7. This
  makes patch reviewing easier. Suggested by David.

  Take Wei Yang's fixing patch out to post alone, since it has been
  reviewed and acked by people. Suggested by Andrew.

  Fix a code comment mistake in the current patch 2/7. Found out by
  Wei Yang during reviewing.

Baoquan He (5):
  mm/sparse.c: introduce new function fill_subsection_map()
  mm/sparse.c: introduce a new function clear_subsection_map()
  mm/sparse.c: only use subsection map in VMEMMAP case
  mm/sparse.c: add note about only VMEMMAP supporting sub-section
    support
  mm/sparse.c: move subsection_map related functions together

 include/linux/mmzone.h |   2 +
 mm/sparse.c            | 136 ++++++++++++++++++++++++++++-------------
 2 files changed, 95 insertions(+), 43 deletions(-)

-- 
2.17.2