It's necessary to monitor lockless pagetable walks, in order to avoid doing
THP splitting/collapsing during them.
On powerpc, we need to do some lockless pagetable walks from functions
that already have disabled interrupts, specially from real mode with
MSR[EE=0].
In these contexts, disabling/enabling interrupts can be very troubling.
So, this arch-specific implementation features functions with an extra
argument that allows interrupt enable/disable to be skipped:
__begin_lockless_pgtbl_walk() and __end_lockless_pgtbl_walk().
Functions similar to the generic ones are also exported, by calling
the above functions with parameter *able_irq = false.
While there is no config option, the method is disabled and these functions
are only doing what was already needed to lockless pagetable walks
(disabling interrupt). A memory barrier was also added just to make sure
there is no speculative read outside the interrupt disabled area.
Signed-off-by: Leonardo Bras <leonardo@xxxxxxxxxxxxx>
---
arch/powerpc/include/asm/book3s/64/pgtable.h | 9 ++
arch/powerpc/mm/book3s64/pgtable.c | 117 +++++++++++++++++++
2 files changed, 126 insertions(+)
diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
index b01624e5c467..8330b35cd28d 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
@@ -1372,5 +1372,14 @@ static inline bool pgd_is_leaf(pgd_t pgd)
return !!(pgd_raw(pgd) & cpu_to_be64(_PAGE_PTE));
}
+#define __HAVE_ARCH_LOCKLESS_PGTBL_WALK_CONTROL
+unsigned long begin_lockless_pgtbl_walk(struct mm_struct *mm);
+unsigned long __begin_lockless_pgtbl_walk(struct mm_struct *mm,
+ bool disable_irq);
+void end_lockless_pgtbl_walk(struct mm_struct *mm, unsigned long irq_mask);
+void __end_lockless_pgtbl_walk(struct mm_struct *mm, unsigned long irq_mask,
+ bool enable_irq);
+int running_lockless_pgtbl_walk(struct mm_struct *mm);
+
#endif /* __ASSEMBLY__ */
#endif /* _ASM_POWERPC_BOOK3S_64_PGTABLE_H_ */
diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 75483b40fcb1..ae557fdce9a3 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -98,6 +98,123 @@ void serialize_against_pte_lookup(struct mm_struct *mm)
smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
}
+/*
+ * Counting method to monitor lockless pagetable walks:
+ * Uses begin_lockless_pgtbl_walk and end_lockless_pgtbl_walk to track the
+ * number of lockless pgtable walks happening, and
+ * running_lockless_pgtbl_walk to return this value.
+ */
+
+/* begin_lockless_pgtbl_walk: Must be inserted before a function call that does
+ * lockless pagetable walks, such as __find_linux_pte().
+ * This version allows setting disable_irq=false, so irqs are not touched, which
+ * is quite useful for running when ints are already disabled (like real-mode)
+ */
+
+inline unsigned long __begin_lockless_pgtbl_walk(struct mm_struct *mm,
+ bool disable_irq)
+{
+ unsigned long irq_mask = 0;
+
+ if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
+ atomic_inc(&mm->lockless_pgtbl_walkers);
+
+ /*
+ * Interrupts must be disabled during the lockless page table walk.
+ * That's because the deleting or splitting involves flushing TLBs,
+ * which in turn issues interrupts, that will block when disabled.
+ *
+ * When this function is called from realmode with MSR[EE=0],
+ * it's not needed to touch irq, since it's already disabled.
+ */
+ if (disable_irq)
+ local_irq_save(irq_mask);
+
+ /*
+ * This memory barrier pairs with any code that is either trying to
+ * delete page tables, or split huge pages. Without this barrier,
+ * the page tables could be read speculatively outside of interrupt
+ * disabling or reference counting.
+ */
+ smp_mb();
+
+ return irq_mask;
+}
+EXPORT_SYMBOL(__begin_lockless_pgtbl_walk);
+
+/* begin_lockless_pgtbl_walk: Must be inserted before a function call that does
+ * lockless pagetable walks, such as __find_linux_pte().
+ * This version is used by generic code, and always assume irqs being disabled
+ */
+unsigned long begin_lockless_pgtbl_walk(struct mm_struct *mm)
+{
+ return __begin_lockless_pgtbl_walk(mm, true);
+}
+EXPORT_SYMBOL(begin_lockless_pgtbl_walk);
+
+/*
+ * __end_lockless_pgtbl_walk: Must be inserted after the last use of a pointer
+ * returned by a lockless pagetable walk, such as __find_linux_pte()
+ * This version allows setting enable_irq=false, so irqs are not touched, which
+ * is quite useful for running when ints are already disabled (like real-mode)
+ */
+inline void __end_lockless_pgtbl_walk(struct mm_struct *mm,
+ unsigned long irq_mask, bool enable_irq)
+{
+ /*
+ * This memory barrier pairs with any code that is either trying to
+ * delete page tables, or split huge pages. Without this barrier,
+ * the page tables could be read speculatively outside of interrupt
+ * disabling or reference counting.
+ */
+ smp_mb();
+
+ /*
+ * Interrupts must be disabled during the lockless page table walk.
+ * That's because the deleting or splitting involves flushing TLBs,
+ * which in turn issues interrupts, that will block when disabled.
+ *
+ * When this function is called from realmode with MSR[EE=0],
+ * it's not needed to touch irq, since it's already disabled.
+ */
+ if (enable_irq)
+ local_irq_restore(irq_mask);
+
+ if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
+ atomic_dec(&mm->lockless_pgtbl_walkers);
+}
+EXPORT_SYMBOL(__end_lockless_pgtbl_walk);
+
+/*
+ * end_lockless_pgtbl_walk: Must be inserted after the last use of a pointer
+ * returned by a lockless pagetable walk, such as __find_linux_pte()
+ * This version is used by generic code, and always assume irqs being enabled
+ */
+
+void end_lockless_pgtbl_walk(struct mm_struct *mm, unsigned long irq_mask)
+{
+ __end_lockless_pgtbl_walk(mm, irq_mask, true);
+}
+EXPORT_SYMBOL(end_lockless_pgtbl_walk);
+
+/*
+ * running_lockless_pgtbl_walk: Returns the number of lockless pagetable walks
+ * currently running. If it returns 0, there is no running pagetable walk, and
+ * THP split/collapse can be safely done. This can be used to avoid more
+ * expensive approaches like serialize_against_pte_lookup()
+ */
+int running_lockless_pgtbl_walk(struct mm_struct *mm)
+{
+ if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
+ return atomic_read(&mm->lockless_pgtbl_walkers);
+
+ /* If disabled, must return > 0, so it fallback to sync method
+ * (serialize_against_pte_lookup)
+ */
+ return 1;
+}
+EXPORT_SYMBOL(running_lockless_pgtbl_walk);
+
/*
* We use this to invalidate a pmdp entry before switching from a
* hugepte to regular pmd entry.
--
2.20.1
