Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 22 May 2013 23:36:45 +0300

On Thu, May 16, 2013 at 08:40:41PM +0200, Peter Zijlstra wrote:
> On Thu, May 16, 2013 at 02:16:10PM +0300, Michael S. Tsirkin wrote:
> > There are several ways to make sure might_fault
> > calling function does not sleep.
> > One is to use it on kernel or otherwise locked memory - apparently
> > nfs/sunrpc does this. As noted by Ingo, this is handled by the
> > migh_fault() implementation in mm/memory.c but not the one in
> > linux/kernel.h so in the current code might_fault() schedules
> > differently depending on CONFIG_PROVE_LOCKING, which is an undesired
> > semantical side effect.
> > 
> > Another is to call pagefault_disable: in this case the page fault
> > handler will go to fixups processing and we get an error instead of
> > sleeping, so the might_sleep annotation is a false positive.
> > vhost driver wants to do this now in order to reuse socket ops
> > under a spinlock (and fall back on slower thread handler
> > on error).
> 
> Are you using the assumption that spin_lock() implies preempt_disable() implies
> pagefault_disable()? Note that this assumption isn't valid for -rt where the
> spinlock becomes preemptible but we'll not disable pagefaults.
> 
> > Address both issues by:
> > 	- dropping the unconditional call to might_sleep
> > 	  from the fast might_fault code in linux/kernel.h
> > 	- checking for pagefault_disable() in the
> > 	  CONFIG_PROVE_LOCKING implementation
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >  include/linux/kernel.h |  1 -
> >  mm/memory.c            | 14 +++++++++-----
> >  2 files changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index e96329c..322b065 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -198,7 +198,6 @@ void might_fault(void);
> >  #else
> >  static inline void might_fault(void)
> >  {
> > -	might_sleep();
> 
> This removes potential resched points for PREEMPT_VOLUNTARY -- was that
> intentional?

OK so I'm thinking of going back to this idea:
it has the advantage of being very simple,
and just might make some workloads faster
if they do lots of copy_XX_user in a loop.

Will have to be tested of course - anyone
has objections?

> >  }
> >  #endif
> >  
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 6dc1882..1b8327b 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4222,13 +4222,17 @@ void might_fault(void)
> >  	if (segment_eq(get_fs(), KERNEL_DS))
> >  		return;
> >  
> > -	might_sleep();
> >  	/*
> > -	 * 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.
> > +	 * It would be nicer to annotate paths which are under preempt_disable
> > +	 * but not under pagefault_disable, however that requires a new flag
> > +	 * for differentiating between the two.
> 
> -rt has this, pagefault_disable() doesn't change the preempt count but pokes
> at task_struct::pagefault_disable.
> 
> >  	 */
> > -	if (!in_atomic() && current->mm)
> > +	if (in_atomic())
> > +		return;
> > +
> > +	might_sleep();
> > +
> > +	if (current->mm)
> >  		might_lock_read(&current->mm->mmap_sem);
> >  }
> >  EXPORT_SYMBOL(might_fault);
> > -- 
> > MST