[PATCH v3 0/7] mm: Introduce for_each_valid_pfn()

[David Woodhouse]

There are cases where a naïve loop over a PFN range, calling pfn_valid() on
each one, is horribly inefficient. Ruihan Li reported the case where
memmap_init() iterates all the way from zero to a potentially large value
of ARCH_PFN_OFFSET, and we at Amazon found the reserve_bootmem_region()
one as it affects hypervisor live update. Others are more cosmetic.

By introducing a for_each_valid_pfn() helper it can optimise away a lot
of pointless calls to pfn_valid(), skipping immediately to the next
valid PFN and also skipping *all* checks within a valid (sub)region
according to the granularity of the memory model in use.

https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/for_each_valid_pfn

v3: 
 • Fold the 'optimised' SPARSEMEM implementation into the original patch
 • Drop the use of (-1) as end marker, and use end_pfn instead.
 • Drop unused first_valid_pfn() helper for FLATMEM implementation
 • Add use case in memmap_init() from discussion at 
   https://lore.kernel.org/linux-mm/20250419122801.1752234-1-lrh2000@pku.edu.cn/

v2 [RFC]: https://lore.kernel.org/linux-mm/20250404155959.3442111-1-dwmw2@infradead.org/
 • Revised implementations with feedback from Mike
 • Add a few more use cases

v1 [RFC]: https://lore.kernel.org/linux-mm/20250402201841.3245371-1-dwmw2@infradead.org/
 • First proof of concept

David Woodhouse (7):
      mm: Introduce for_each_valid_pfn() and use it from reserve_bootmem_region()
      mm: Implement for_each_valid_pfn() for CONFIG_FLATMEM
      mm: Implement for_each_valid_pfn() for CONFIG_SPARSEMEM
      mm, PM: Use for_each_valid_pfn() in kernel/power/snapshot.c
      mm, x86: Use for_each_valid_pfn() from __ioremap_check_ram()
      mm: Use for_each_valid_pfn() in memory_hotplug
      mm/mm_init: Use for_each_valid_pfn() in init_unavailable_range()

 arch/x86/mm/ioremap.c              |  7 ++-
 include/asm-generic/memory_model.h | 26 ++++++++++-
 include/linux/mmzone.h             | 88 ++++++++++++++++++++++++++++++++++++++
 kernel/power/snapshot.c            | 42 +++++++++---------
 mm/memory_hotplug.c                |  8 +---
 mm/mm_init.c                       | 29 +++++--------
 6 files changed, 149 insertions(+), 51 deletions(-)