[PATCH v12 0/4] fix double page fault in cow_user_page for pfn mapping

From: Jia He
Date: Fri Oct 11 2019 - 10:09:53 EST

When we tested pmdk unit test vmmalloc_fork TEST1 in arm64 guest, there
will be a double page fault in __copy_from_user_inatomic of cow_user_page.

As told by Catalin: "On arm64 without hardware Access Flag, copying from
user will fail because the pte is old and cannot be marked young. So we
always end up with zeroed page after fork() + CoW for pfn mappings. we
don't always have a hardware-managed access flag on arm64."

refine PATCH 01, remove the !! since C languages can convert unsigned
to bool (Catalin)
refine cpu_has_hw_af in PATCH 01(Will Deacon, Suzuki)
change the default return value to true in arch_faults_on_old_pte
add PATCH 03 for overriding arch_faults_on_old_pte(false) on x86
add r-b from Catalin and a-b from Kirill in PATCH 03
remoe Reported-by in PATCH 01
v9: refactor cow_user_page for indention optimization (Catalin)
hold the ptl longer (Catalin)
v8: change cow_user_page's return type (Matthew)
v7: s/pte_spinlock/pte_offset_map_lock (Kirill)
v6: fix error case of returning with spinlock taken (Catalin)
move kmap_atomic to avoid handling kunmap_atomic
v5: handle the case correctly when !pte_same
fix kbuild test failed
v4: introduce cpu_has_hw_af (Suzuki)
bail out if !pte_same (Kirill)
v3: add vmf->ptl lock/unlock (Kirill A. Shutemov)
add arch_faults_on_old_pte (Matthew, Catalin)
v2: remove FAULT_FLAG_WRITE when setting pte access flag (Catalin)

Jia He (4):
arm64: cpufeature: introduce helper cpu_has_hw_af()
arm64: mm: implement arch_faults_on_old_pte() on arm64
x86/mm: implement arch_faults_on_old_pte() stub on x86
mm: fix double page fault on arm64 if PTE_AF is cleared

arch/arm64/include/asm/cpufeature.h | 14 ++++
arch/arm64/include/asm/pgtable.h | 14 ++++
arch/x86/include/asm/pgtable.h | 6 ++
mm/memory.c | 104 ++++++++++++++++++++++++----
4 files changed, 123 insertions(+), 15 deletions(-)