mm: Prepare decoupling the page fault disabling logic

Add a pagefault_disabled variable to task_struct to allow decoupling
the pagefault-disabled logic from the preempt count.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/sched.h

@@ -1360,6 +1360,7 @@ struct task_struct {
 	/* mutex deadlock detection */
 	struct mutex_waiter *blocked_on;
 #endif
+	int pagefault_disabled;
 #ifdef CONFIG_TRACE_IRQFLAGS
 	unsigned int irq_events;
 	int hardirqs_enabled;

include/linux/uaccess.h

@@ -6,37 +6,10 @@
 
 /*
  * These routines enable/disable the pagefault handler in that
- * it will not take any locks and go straight to the fixup table.
- *
- * They have great resemblance to the preempt_disable/enable calls
- * and in fact they are identical; this is because currently there is
- * no other way to make the pagefault handlers do this. So we do
- * disable preemption but we don't necessarily care about that.
- */
-static inline void pagefault_disable(void)
-{
-	inc_preempt_count();
-	/*
-	 * make sure to have issued the store before a pagefault
-	 * can hit.
+ * it will not take any MM locks and go straight to the fixup table.
  */
-	barrier();
-}
-
-static inline void pagefault_enable(void)
-{
-	/*
-	 * make sure to issue those last loads/stores before enabling
-	 * the pagefault handler again.
-	 */
-	barrier();
-	dec_preempt_count();
-	/*
-	 * make sure we do..
-	 */
-	barrier();
-	preempt_check_resched();
-}
+extern void pagefault_disable(void);
+extern void pagefault_enable(void);
 
 #ifndef ARCH_HAS_NOCACHE_UACCESS
 

kernel/fork.c

@@ -1080,6 +1080,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	p->hardirq_context = 0;
 	p->softirq_context = 0;
 #endif
+	p->pagefault_disabled = 0;
 #ifdef CONFIG_LOCKDEP
 	p->lockdep_depth = 0; /* no locks held yet */
 	p->curr_chain_key = 0;

mm/memory.c

@@ -2955,6 +2955,35 @@ unlock:
 	return 0;
 }
 
+void pagefault_disable(void)
+{
+	inc_preempt_count();
+	current->pagefault_disabled++;
+	/*
+	 * make sure to have issued the store before a pagefault
+	 * can hit.
+	 */
+	barrier();
+}
+EXPORT_SYMBOL(pagefault_disable);
+
+void pagefault_enable(void)
+{
+	/*
+	 * make sure to issue those last loads/stores before enabling
+	 * the pagefault handler again.
+	 */
+	barrier();
+	current->pagefault_disabled--;
+	dec_preempt_count();
+	/*
+	 * make sure we do..
+	 */
+	barrier();
+	preempt_check_resched();
+}
+EXPORT_SYMBOL(pagefault_enable);
+
 /*
  * By the time we get here, we already hold the mm semaphore
  */