Re: PFs on pages pinned with get_user_pages()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2009-01-29 at 15:41 +0100, Frank Mehnert wrote:
> On Thursday 29 January 2009, Peter Zijlstra wrote:
> > On Thu, 2009-01-29 at 15:02 +0100, Frank Mehnert wrote:
> > > I'm one of the VirtualBox developers. We are trying to fix the annoying
> > > kerneloops warning 'BUG: sleeping function called from invalid context'
> > > reported by the Fedora folks. This warning occurs when do_swap_page()
> > > calls lock_page() and in_atomic() returns true.
> > >
> > > This warning appears when we touch into memory which is pinned with
> > > get_user_pages(). In VT-x/AMD-V mode we are executing some code in the
> > > context of the Linux kernel. To prevent scheduling of the current CPU
> > > core we disable the interripts. preempt_disable() would be probably the
> > > better choice but this would oops as well if CONFIG_PREEMPT is enabled.
> >
> > but to get there, you'd have to have called handle_mm_fault() which
> > requires the mmap_sem, which should also give that might_sleep()
> > warning.
> 
> The stacktrace is
> 
>   __might_sleep()
>   lock_page()
>   handle_mm_fault()
>   do_page_fault()
>   error_code
> 
> So yes, handle_mm_fault() is called. But I assume that down_read_trylock()
> succeeded before we were forced to call down_read().
> 
> > That aside, is there any reason you have to avoid scheduling? Otherwise
> > I would just allow so and be done with it.
> 
> The reason is that our code expects that to ensure syncing of the CPU
> state with the saved state. I fear it is quite difficult to change that...

Ah, is that what KVM uses the preempt notifiers for? Could you too?