Re: [PATCH] mm, arch: Complete pagefault_disable abstraction

Peter Zijlstra
2011-10-05T13:10:31Z

For reference, here's the -rt patch that goes on top:

---
Subject: rt: Preemptable pagefault_disable()
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Wed Oct 05 14:20:57 CEST 2011

Implement a preemptable pagefault_disable() by keeping a per-task
pagefault_disabled counter.

This allows disabling of the pagefault handler (and thus avoiding the
recursive fault/mmap_sem issues) without disabling preemption.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/sched.h | 9 ++++++++-
include/linux/uaccess.h | 5 +++++
kernel/fork.c | 3 +++
mm/memory.c | 24 ++++++++++++++++++++++++
4 files changed, 40 insertions(+), 1 deletion(-)

Index: linux-2.6/include/linux/uaccess.h
===================================================================
--- linux-2.6.orig/include/linux/uaccess.h
+++ linux-2.6/include/linux/uaccess.h
@@ -4,6 +4,7 @@
#include <linux/preempt.h>
#include <asm/uaccess.h>

+#ifndef CONFIG_PREEMPT_RT_FULL
/*
* These routines enable/disable the pagefault handler in that
* it will not take any locks and go straight to the fixup table.
@@ -37,6 +38,10 @@ static inline void pagefault_enable(void
barrier();
preempt_check_resched();
}
+#else
+extern void pagefault_disable(void);
+extern void pagefault_enable(void);
+#endif

#ifndef ARCH_HAS_NOCACHE_UACCESS

Index: linux-2.6/kernel/fork.c
===================================================================
--- linux-2.6.orig/kernel/fork.c
+++ linux-2.6/kernel/fork.c
@@ -1200,6 +1200,9 @@ static struct task_struct *copy_process(
p->hardirq_context = 0;
p->softirq_context = 0;
#endif
+#ifdef CONFIG_PREEMPT_RT_FULL
+ p->pagefault_disabled = 0;
+#endif
#ifdef CONFIG_LOCKDEP
p->lockdep_depth = 0; /* no locks held yet */
p->curr_chain_key = 0;
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -3436,6 +3436,30 @@ int handle_pte_fault(struct mm_struct *m
return 0;
}

+#ifdef CONFIG_PREEMPT_RT_FULL
+void pagefault_disable(void)
+{
+ current->pagefault_disabled++;
+ /*
+ * make sure to have issued the store before a pagefault
+ * can hit.
+ */
+ barrier();
+}
+EXPORT_SYMBOL_GPL(pagefault_disable);
+
+void pagefault_enable(void)
+{
+ /*
+ * make sure to issue those last loads/stores before enabling
+ * the pagefault handler again.
+ */
+ barrier();
+ current->pagefault_disabled--;
+}
+EXPORT_SYMBOL_GPL(pagefault_enable);
+#endif
+
/*
* By the time we get here, we already hold the mm semaphore
*/
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1434,6 +1434,9 @@ struct task_struct {
/* mutex deadlock detection */
struct mutex_waiter *blocked_on;
#endif
+#ifdef CONFIG_PREEMPT_RT_FULL
+ int pagefault_disabled;
+#endif
#ifdef CONFIG_TRACE_IRQFLAGS
unsigned int irq_events;
unsigned long hardirq_enable_ip;
@@ -1578,7 +1581,11 @@ struct task_struct {

static inline bool pagefault_disabled(void)
{
- return in_atomic();
+ return in_atomic()
+#ifdef CONFIG_PREEMPT_RT_FULL
+ || current->pagefault_disabled
+#endif
+ ;
}

/*