[RFC,0/4] arm64/mm: remap crash kernel with base pages even if rodata_full disabled

Message

Mike Rapoport Aug. 1, 2022, 8:04 a.m. UTC

From: Mike Rapoport <[e-mail]>

Hi,

There were several rounds of discussion how to remap with base pages only
the crash kernel area, the latest one here:

https://lore.kernel.org/all/[e-mail]

and this is my attempt to allow having both large pages in the linear map
and protection for the crash kernel memory.

For server systems it is important to protect crash kernel memory for
post-mortem analysis, and for that protection to work the crash kernel
memory should be mapped with base pages in the linear map. 

On the systems with ZONE_DMA/DMA32 enabled, crash kernel reservation
happens after the linear map is created and the current code forces using
base pages for the entire linear map, which results in performance
degradation.

These patches enable remapping of the crash kernel area with base pages
while keeping large pages in the rest of the linear map.

The idea is to align crash kernel reservation to PUD boundaries, remap that
PUD and then free the extra memory.

For now the remapping does not deal with the case when crash kernel base is
specified, but this won't be a problem to add if the idea is generally
acceptable.

Mike Rapoport (4):
  arm64: introduce have_zone_dma() helper
  arm64/mmu: drop _hotplug from unmap_hotplug_* function names
  arm64/mmu: move helpers for hotplug page tables freeing close to callers
  arm64/mm: remap crash kernel with base pages even if rodata_full disabled

 arch/arm64/include/asm/memory.h |   8 +++
 arch/arm64/include/asm/mmu.h    |   2 +
 arch/arm64/mm/init.c            |  44 ++++++++++--
 arch/arm64/mm/mmu.c             | 116 ++++++++++++++++++++------------
 4 files changed, 122 insertions(+), 48 deletions(-)