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SubjectRe: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep
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.

No, I was not assuming that. What I'm trying to say is that a caller
that does something like this under a spinlock:
preempt_disable
pagefault_disable
error = copy_to_user
pagefault_enable
preempt_enable_no_resched

is not doing anything wrong and should not get a warning,
as long as error is handled correctly later.
Right?

> > 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 <mst@redhat.com>
> > ---
> > 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?

No it's a bug. Thanks for pointing this out.
OK so I guess it should be might_sleep_if(!in_atomic())
and this means might_fault would have to move from linux/kernel.h to
linux/uaccess.h, since in_atomic() is in linux/hardirq.h

Makes sense?

> > }
> > #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.

Good to know.

So maybe we can import this at least for CONFIG_PROVE_LOCKING?
To make the patch smaller I'd prefer doing both for now,
this way this patchset does not have to poke in too many
mm internals.
I can try doing that - unless
someone else has plans to merge this part soon anyway?

> > */
> > - 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


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