[PATCH v1 01/15] uaccess: count pagefault_disable() levels in pagefault_disabled

David Hildenbrand
2015-05-11T15:52:57Z

Until now, pagefault_disable()/pagefault_enabled() used the preempt
count to track whether in an environment with pagefaults disabled (can
be queried via in_atomic()).

This patch introduces a separate counter in task_struct to count the
level of pagefault_disable() calls. We'll keep manipulating the preempt
count to retain compatibility to existing pagefault handlers.

It is now possible to verify whether in a pagefault_disable() envionment
by calling pagefault_disabled(). In contrast to in_atomic() it will not
be influenced by preempt_enable()/preempt_disable().

This patch is based on a patch from Ingo Molnar.

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
---
include/linux/sched.h | 1 +
include/linux/uaccess.h | 36 +++++++++++++++++++++++++++++-------
kernel/fork.c | 3 +++
3 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index af01e3c..6df2790 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1792,6 +1792,7 @@ struct task_struct {
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
unsigned long task_state_change;
#endif
+ int pagefault_disabled;
};

/* Future-safe accessor for struct task_struct's cpus_allowed. */
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index ecd3319..23290cc 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -2,20 +2,36 @@
#define __LINUX_UACCESS_H__

#include <linux/preempt.h>
+#include <linux/sched.h>
#include <asm/uaccess.h>

+static __always_inline void pagefault_disabled_inc(void)
+{
+ current->pagefault_disabled++;
+}
+
+static __always_inline void pagefault_disabled_dec(void)
+{
+ current->pagefault_disabled--;
+ WARN_ON(current->pagefault_disabled < 0);
+}
+
/*
- * These routines enable/disable the pagefault handler in that
- * it will not take any locks and go straight to the fixup table.
+ * These routines enable/disable the pagefault handler. If disabled, it will
+ * not take any locks and go straight to the fixup table.
+ *
+ * We increase the preempt and the pagefault count, to be able to distinguish
+ * whether we run in simple atomic context or in a real pagefault_disable()
+ * context.
+ *
+ * For now, after pagefault_disabled() has been called, we run in atomic
+ * context. User access methods will not sleep.
*
- * 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)
{
preempt_count_inc();
+ pagefault_disabled_inc();
/*
* make sure to have issued the store before a pagefault
* can hit.
@@ -25,18 +41,24 @@ static inline void pagefault_disable(void)

static inline void pagefault_enable(void)
{
-#ifndef CONFIG_PREEMPT
/*
* make sure to issue those last loads/stores before enabling
* the pagefault handler again.
*/
barrier();
+ pagefault_disabled_dec();
+#ifndef CONFIG_PREEMPT
preempt_count_dec();
#else
preempt_enable();
#endif
}

+/*
+ * Is the pagefault handler disabled? If so, user access methods will not sleep.
+ */
+#define pagefault_disabled() (current->pagefault_disabled != 0)
+
#ifndef ARCH_HAS_NOCACHE_UACCESS

static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
diff --git a/kernel/fork.c b/kernel/fork.c
index 2e67086..0bb88b5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1393,6 +1393,9 @@ 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;
--
2.1.4