[RFC 0/4] mm: Introduce lazy exec permission setting on a page

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

Setting an exec permission on a page normally triggers I-cache invalidation
which might be expensive. I-cache invalidation is not mandatory on a given
page if there is no immediate exec access on it. Non-fault modification of
user page table from generic memory paths like migration can be improved if
setting of the exec permission on the page can be deferred till actual use.
There was a performance report [1] which highlighted the problem.

This introduces [pte|pmd]_mklazyexec() which clears the exec permission on
a page during migration. This exec permission deferral must be enabled back
with maybe_[pmd]_mkexec() during exec page fault (FAULT_FLAG_INSTRUCTION)
if the corresponding VMA contains exec flag (VM_EXEC).

This framework is encapsulated under CONFIG_ARCH_SUPPORTS_LAZY_EXEC so that
non-subscribing architectures don't take any performance hit. For now only
generic memory migration path will be using this framework but later it can
be extended to other generic memory paths as well.

This enables CONFIG_ARCH_SUPPORTS_LAZY_EXEC on arm64 and defines required
helper functions in this regard while changing ptep_set_access_flags() to
allow non-exec to exec transition.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2018-December/620357.html

Anshuman Khandual (4):
  mm: Introduce lazy exec permission setting on a page
  arm64/mm: Identify user level instruction faults
  arm64/mm: Allow non-exec to exec transition in ptep_set_access_flags()
  arm64/mm: Enable ARCH_SUPPORTS_LAZY_EXEC

 arch/arm64/Kconfig               |  1 +
 arch/arm64/include/asm/pgtable.h | 17 +++++++++++++++++
 arch/arm64/mm/fault.c            | 22 ++++++++++++++--------
 include/asm-generic/pgtable.h    | 12 ++++++++++++
 include/linux/mm.h               | 26 ++++++++++++++++++++++++++
 mm/Kconfig                       |  9 +++++++++
 mm/huge_memory.c                 |  5 +++++
 mm/hugetlb.c                     |  2 ++
 mm/memory.c                      |  4 ++++
 mm/migrate.c                     |  2 ++
 10 files changed, 92 insertions(+), 8 deletions(-)

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2.7.4