This is v2 post. V1 can be found here:

In sparse_init(), two temporary pointer arrays, usemap_map and map_map
are allocated with the size of NR_MEM_SECTIONS. They are used to store
each memory section's usemap and mem map if marked as present. In
5-level paging mode, this will cost 512M memory though they will be
released at the end of sparse_init(). System with few memory, like
kdump kernel which usually only has about 256M, will fail to boot
because of allocation failure if CONFIG_X86_5LEVEL=y.

In this patchset, optimize the memmap allocation code to only use
usemap_map and map_map with the size of nr_present_sections. This
makes kdump kernel boot up with normal crashkernel='' setting when

Baoquan He (3):
  mm/sparse: Add a static variable nr_present_sections
  mm/sparsemem: Defer the ms->section_mem_map clearing
  mm/sparse: Optimize memmap allocation during sparse_init()

 mm/sparse-vmemmap.c |  9 +++++----
 mm/sparse.c         | 54 +++++++++++++++++++++++++++++++++++------------------
 2 files changed, 41 insertions(+), 22 deletions(-)


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