x86/mm/tlb: Always use lazy TLB mode

Now that CPUs in lazy TLB mode no longer receive TLB shootdown IPIs, except
at page table freeing time, and idle CPUs will no longer get shootdown IPIs
for things like mprotect and madvise, we can always use lazy TLB mode.

Tested-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: efault@gmx.de
Cc: kernel-team@fb.com
Cc: luto@kernel.org
Link: http://lkml.kernel.org/r/20180716190337.26133-7-riel@surriel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/tlb.c

@@ -379,20 +379,7 @@ void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
 	if (this_cpu_read(cpu_tlbstate.loaded_mm) == &init_mm)
 		return;
 
-	if (tlb_defer_switch_to_init_mm()) {
-		/*
-		 * There's a significant optimization that may be possible
-		 * here.  We have accurate enough TLB flush tracking that we
-		 * don't need to maintain coherence of TLB per se when we're
-		 * lazy.  We do, however, need to maintain coherence of
-		 * paging-structure caches.  We could, in principle, leave our
-		 * old mm loaded and only switch to init_mm when
-		 * tlb_remove_page() happens.
-		 */
-		this_cpu_write(cpu_tlbstate.is_lazy, true);
-	} else {
-		switch_mm(NULL, &init_mm, NULL);
-	}
+	this_cpu_write(cpu_tlbstate.is_lazy, true);
 }
 
 /*