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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 27 Nov 2014 19:24:49 +0200

On Thu, Nov 27, 2014 at 06:10:17PM +0100, David Hildenbrand wrote:
> 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 <[email-masked]>
> ---
>  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);

This __FILE__/__FILE__ will always point at kernel.h

You want a macro to wrap this up.

> +}
>  #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