Some architectures invoke pagetable_alloc() or __get_free_pages()
with preemption disabled.
For example, in arm64, linear_map_split_to_ptes() calls pagetable_alloc()
while spliting block entry to ptes and __kpti_install_ng_mappings()
calls __get_free_pages() to create kpti pagetable.

Under PREEMPT_RT, calling pagetable_alloc() with
preemption disabled is not allowed, because it may acquire
a spin lock that becomes sleepable on RT, potentially
causing a sleep during page allocation.

Since above two functions is called as callback of stop_machine()
where its callback is called in preemption disabled,
They could make a potential problem. (sleeping in preemption disabled).

To address this, introduce pagetable_alloc_nolock() API.

Yeoreum Yun (2):
  mm: introduce pagetable_alloc_nolock()
  arm64: mmu: use pagetable_alloc_nolock() while stop_machine()

 arch/arm64/mm/mmu.c  | 23 ++++++++++++++++++-----
 include/linux/mm.h   | 18 ++++++++++++++++++
 kernel/bpf/stream.c  |  2 +-
 kernel/bpf/syscall.c |  2 +-
 mm/page_alloc.c      | 10 +++-------
 5 files changed, 41 insertions(+), 14 deletions(-)

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