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Diffstat (limited to 'arch/x86/include/asm/mmu_context.h')
-rw-r--r--arch/x86/include/asm/mmu_context.h20
1 files changed, 19 insertions, 1 deletions
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 40269a2bf6f9..4b75d591eb5e 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -130,7 +130,25 @@ static inline void arch_bprm_mm_init(struct mm_struct *mm,
static inline void arch_unmap(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long start, unsigned long end)
{
- mpx_notify_unmap(mm, vma, start, end);
+ /*
+ * mpx_notify_unmap() goes and reads a rarely-hot
+ * cacheline in the mm_struct. That can be expensive
+ * enough to be seen in profiles.
+ *
+ * The mpx_notify_unmap() call and its contents have been
+ * observed to affect munmap() performance on hardware
+ * where MPX is not present.
+ *
+ * The unlikely() optimizes for the fast case: no MPX
+ * in the CPU, or no MPX use in the process. Even if
+ * we get this wrong (in the unlikely event that MPX
+ * is widely enabled on some system) the overhead of
+ * MPX itself (reading bounds tables) is expected to
+ * overwhelm the overhead of getting this unlikely()
+ * consistently wrong.
+ */
+ if (unlikely(cpu_feature_enabled(X86_FEATURE_MPX)))
+ mpx_notify_unmap(mm, vma, start, end);
}
#endif /* _ASM_X86_MMU_CONTEXT_H */