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From: David Hildenbrand <>
Subject: [PATCH v1 3/5] mm, uaccess: trigger might_sleep() in might_fault() when pagefaults are disabled
Date: Fri,  5 Dec 2014 12:18:07 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Commit 662bbcb2747c ("mm, sched: Allow uaccess in atomic with
pagefault_disable()") 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
    rc = copy_to_user(...)

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:
    rc = copy_to_user(...)

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 pagefault_disabled(), we can use it to distingusih whether user
acces functions might sleep.

Convert might_fault() into a makro that calls __might_fault(), to allow proper
file + line messages in case of a might_sleep() warning. We can't move the code
directly into kernel.h for now, as that results in ugly header recursions we
can't avoid for now.

Signed-off-by: David Hildenbrand <>
 include/linux/kernel.h |  3 ++-
 mm/memory.c            | 19 +++++++------------
 2 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3d770f55..aa0907e 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -227,7 +227,8 @@ static inline u32 reciprocal_scale(u32 val, u32 ep_ro)
 #if defined(CONFIG_MMU) && \
-void might_fault(void);
+#define might_fault() __might_fault(__FILE__, __LINE__)
+void __might_fault(const char *file, int line);
 static inline void might_fault(void) { }
diff --git a/mm/memory.c b/mm/memory.c
index d5f2ae9..54dde04 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3700,7 +3700,7 @@ void print_vma_addr(char *prefix, unsigned long ip)
-void might_fault(void)
+void __might_fault(const char *file, int line)
 	 * Some code (nfs/sunrpc) uses socket ops on kernel memory while
@@ -3710,21 +3710,16 @@ void might_fault(void)
 	if (segment_eq(get_fs(), KERNEL_DS))
-	/*
-	 * 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())
+	if (unlikely(!pagefault_disabled())) {
+		__might_sleep(file, line, 0);
-	__might_sleep(__FILE__, __LINE__, 0);
+	}
 	if (current->mm)

  parent reply	other threads:[~2014-12-05 11:18 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-12-05 11:18 [PATCH v1 0/5] Reenable might_sleep() checks for might_fault() David Hildenbrand
2014-12-05 11:18 ` [PATCH v1 1/5] uaccess: add pagefault_count to thread_info David Hildenbrand
2014-12-08 13:12   ` Christian Borntraeger
2014-12-08 13:24     ` David Hildenbrand
2014-12-05 11:18 ` [PATCH v1 2/5] uaccess: count pagefault_disable() levels in pagefault_count David Hildenbrand
2014-12-05 11:18 ` David Hildenbrand [this message]
2014-12-05 11:45   ` [PATCH v1 3/5] mm, uaccess: trigger might_sleep() in might_fault() when pagefaults are disabled Heiko Carstens
2014-12-05 11:46     ` David Hildenbrand
2014-12-05 12:08       ` David Laight
2014-12-05 13:30         ` David Hildenbrand
2014-12-05 11:18 ` [PATCH v1 4/5] uaccess: clearify that uaccess may only sleep if pagefaults are not disabled David Hildenbrand
2014-12-05 11:18 ` [PATCH v1 5/5] uaccess: CONFIG_DEBUG_PAGEFAULT_COUNT to debug pagefault_count David Hildenbrand

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