[PATCH v4 0/3] fix wrong usage of memory allocation APIs under PREEMPT_RT in arm64

[Yeoreum Yun]

Under PREEMPT_RT, calling generic memory allocation/free APIs
(e.x) __get_free_pages(), pgtable_alloc(), free_pages() and etc
with preemption disabled is not allowed, but allow only nolock() APIs series
because it may acquire a spin lock that becomes sleepable on RT,
potentially causing a sleep during page allocation
(See Documentation/core-api/real-time/differences.rst, Memory allocation section).

However, In arm64, __linear_map_split_to_ptes() and
__kpti_install_ng_mappings() called by stopper thread via stop_machine()
use generic memory allocation/free APIs.

This patchset fixes this problem and based on v6.19-rc1

Patch History
==============
from v3 to v4:
  - rebase to v6.19-rc3
  - introduce pgtable_alloc_t
  - following several suggestions from @Ryan Roberts
  - https://lore.kernel.org/all/20251218194750.395301-1-yeoreum.yun@arm.com/

from v2 to v3:
  - remove split-mode and split_args.
    pass proper function pointer while spliting.
  - rename function name.
  - https://lore.kernel.org/all/20251217182007.2345700-1-yeoreum.yun@arm.com/

from v1 to v2:
  - drop pagetable_alloc_nolock()
  - following @Ryan Roberts suggestion.
  - https://lore.kernel.org/all/20251212161832.2067134-1-yeoreum.yun@arm.com/


Yeoreum Yun (3):
  arm64: mmu: introduce pgtable_alloc_t
  arm64: mmu: avoid allocating pages while splitting the linear mapping
  arm64: mmu: avoid allocating pages while installing ng-mapping for
    KPTI

 arch/arm64/mm/mmu.c | 243 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 188 insertions(+), 55 deletions(-)

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