[PATCH RFC 2/2] mm, sched: trigger might_sleep() in might_fault() when pagefaults are disabled

[David Hildenbrand]

Commit 662bbcb2747c2422cf98d3d97619509379eee466 removed might_sleep() checks
for all user access code (that uses might_fault()).

The reason was to disable wrong "sleep in atomic" warnings in the following
scenario:
	pagefault_disable();
	rc = copy_to_user(...);
	pagefault_enable();

Which is valid, as pagefault_disable() increments the preempt counter and
therefore disables the pagefault handler. copy_to_user() will not sleep and return
an invalid return code if a page is not available.

However, as all might_sleep() checks are removed, CONFIG_DEBUG_ATOMIC_SLEEP
would no longer detect the following scenario:
	spin_lock(&lock);
	rc = copy_to_user(...);
	spin_unlock(&lock);

If the kernel is compiled with preemption turned on, the preempt counter would
be incremented and copy_to_user() would never sleep. However, with preemption
turned off, the preempt counter will not be touched, we will therefore sleep in
atomic context. We really want to enable CONFIG_DEBUG_ATOMIC_SLEEP checks for
user access functions again, otherwise horrible deadlocks might be hard to debug.

Root of all evil is that pagefault_disable() acted almost as preempt_disable(),
depending on preemption being turned on/off.

As we now have a fixed pagefault_disable() implementation in place, that uses
own bits in the preempt counter, we can reenable might_sleep() checks.

This patch reverts commit 662bbcb2747c2422cf98d3d97619509379eee466 taking care
of the !MMU optimization and the new pagefault_disabled() check.

Signed-off-by: David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>
---
 include/linux/kernel.h |  9 +++++++--
 mm/memory.c            | 15 ++++-----------
 2 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3d770f55..64b5f93 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -225,9 +225,14 @@ static inline u32 reciprocal_scale(u32 val, u32 ep_ro)
 	return (u32)(((u64) val * ep_ro) >> 32);
 }
 
-#if defined(CONFIG_MMU) && \
-	(defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP))
+#if defined(CONFIG_MMU) && defined(CONFIG_PROVE_LOCKING)
 void might_fault(void);
+#elif defined(CONFIG_MMU) && defined(CONFIG_DEBUG_ATOMIC_SLEEP)
+static inline void might_fault(void)
+{
+	if (unlikely(!pagefault_disabled()))
+		__might_sleep(__FILE__, __LINE__, 0);
+}
 #else
 static inline void might_fault(void) { }
 #endif
diff --git a/mm/memory.c b/mm/memory.c
index 3e50383..0e59db9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3699,7 +3699,7 @@ void print_vma_addr(char *prefix, unsigned long ip)
 	up_read(&mm->mmap_sem);
 }
 
-#if defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)
+#ifdef CONFIG_PROVE_LOCKING
 void might_fault(void)
 {
 	/*
@@ -3711,17 +3711,10 @@ void might_fault(void)
 	if (segment_eq(get_fs(), KERNEL_DS))
 		return;
 
-	/*
-	 * it would be nicer only to annotate paths which are not under
-	 * pagefault_disable, however that requires a larger audit and
-	 * providing helpers like get_user_atomic.
-	 */
-	if (in_atomic())
-		return;
-
-	__might_sleep(__FILE__, __LINE__, 0);
+	if (unlikely(!pagefault_disabled()))
+		__might_sleep(__FILE__, __LINE__, 0);
 
-	if (current->mm)
+	if (!in_atomic() && current->mm)
 		might_lock_read(&current->mm->mmap_sem);
 }
 EXPORT_SYMBOL(might_fault);
-- 
1.8.5.5

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