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 <m...@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?
> }
> #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(¤t->mm->mmap_sem);
> }
> EXPORT_SYMBOL(might_fault);
> --
> MST
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