[RFC 0/2] arm64/mm: Enable THP migration

[Anshuman Khandual <anshuman.khandual@arm.com>]

This series enables THP migration without split on arm64 by subscribing
to ARCH_ENABLE_THP_MIGRATION. Before that it modifies arm64 platform THP
helpers like pmd_present() and pmd_trans_huge() to comply with expected
generic MM semantics as concluded from a previous discussion [1].

Initial THP migration and stress tests look good for various THP sizes. I
will continue testing this further. But meanwhile looking for some early
reviews, feedbacks and suggestions on the approach.

This is based on linux-next tree (next-20190626).

Question:

Instead of directly using PTE_SPECIAL, would it be better to override the
same bit as PMD_SPLITTING and create it's associated helpers to make this
more clear and explicit ?

[1] https://lkml.org/lkml/2018/10/9/220

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

Anshuman Khandual (2):
  arm64/mm: Change THP helpers to comply with generic MM semantics
  arm64/mm: Enable THP migration without split

 arch/arm64/Kconfig               |  4 ++++
 arch/arm64/include/asm/pgtable.h | 32 +++++++++++++++++++++++++++++---
 2 files changed, 33 insertions(+), 3 deletions(-)

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2.7.4