If a process (qemu) with a lot of CPUs (128) try to munmap() a large
chunk of memory (496GB) mapped with THP, it takes an average of 275
seconds, which can cause a lot of problems to the load (in qemu case,
the guest will lock for this time).

Trying to find the source of this bug, I found out most of this time is
spent on serialize_against_pte_lookup(). This function will take a lot
of time in smp_call_function_many() if there is more than a couple CPUs
running the user process. Since it has to happen to all THP mapped, it
will take a very long time for large amounts of memory.

By the docs, serialize_against_pte_lookup() is needed in order to avoid
pmd_t to pte_t casting inside find_current_mm_pte(), or any lockless
pagetable walk, to happen concurrently with THP splitting/collapsing.

It does so by calling a do_nothing() on each CPU in mm->cpu_bitmap[],
after interrupts are re-enabled. 
Since, interrupts are (usually) disabled during lockless pagetable
walk, and serialize_against_pte_lookup will only return after
interrupts are enabled, it is protected.

So, by what I could understand, if there is no lockless pagetable walk
running, there is no need to call serialize_against_pte_lookup().

So, to avoid the cost of running serialize_against_pte_lookup(), I
propose a counter that keeps track of how many find_current_mm_pte()
are currently running, and if there is none, just skip 
smp_call_function_many().

The related functions are:
start_lockless_pgtbl_walk(mm)
	Insert before starting any lockless pgtable walk
end_lockless_pgtbl_walk(mm)
	Insert after the end of any lockless pgtable walk
	(Mostly after the ptep is last used)
running_lockless_pgtbl_walk(mm)
	Returns the number of lockless pgtable walks running


On my workload (qemu), I could see munmap's time reduction from 275
seconds to 418ms.

> Leonardo Bras (11):
>   powerpc/mm: Adds counting method to monitor lockless pgtable walks
>   asm-generic/pgtable: Adds dummy functions to monitor lockless pgtable
>     walks
>   mm/gup: Applies counting method to monitor gup_pgd_range
>   powerpc/mce_power: Applies counting method to monitor lockless pgtbl
>     walks
>   powerpc/perf: Applies counting method to monitor lockless pgtbl walks
>   powerpc/mm/book3s64/hash: Applies counting method to monitor lockless
>     pgtbl walks
>   powerpc/kvm/e500: Applies counting method to monitor lockless pgtbl
>     walks
>   powerpc/kvm/book3s_hv: Applies counting method to monitor lockless
>     pgtbl walks
>   powerpc/kvm/book3s_64: Applies counting method to monitor lockless
>     pgtbl walks
>   powerpc/book3s_64: Enables counting method to monitor lockless pgtbl
>     walk
>   powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing
> 
>  arch/powerpc/include/asm/book3s/64/mmu.h     |  3 +++
>  arch/powerpc/include/asm/book3s/64/pgtable.h |  5 +++++
>  arch/powerpc/kernel/mce_power.c              | 13 ++++++++++---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c          |  2 ++
>  arch/powerpc/kvm/book3s_64_mmu_radix.c       | 20 ++++++++++++++++++--
>  arch/powerpc/kvm/book3s_64_vio_hv.c          |  4 ++++
>  arch/powerpc/kvm/book3s_hv_nested.c          |  8 ++++++++
>  arch/powerpc/kvm/book3s_hv_rm_mmu.c          |  9 ++++++++-
>  arch/powerpc/kvm/e500_mmu_host.c             |  4 ++++
>  arch/powerpc/mm/book3s64/hash_tlb.c          |  2 ++
>  arch/powerpc/mm/book3s64/hash_utils.c        |  7 +++++++
>  arch/powerpc/mm/book3s64/mmu_context.c       |  1 +
>  arch/powerpc/mm/book3s64/pgtable.c           | 20 +++++++++++++++++++-
>  arch/powerpc/perf/callchain.c                |  5 ++++-
>  include/asm-generic/pgtable.h                |  9 +++++++++
>  mm/gup.c                                     |  4 ++++
>  16 files changed, 108 insertions(+), 8 deletions(-)
>