linux-kernel - Re: PFs on pages pinned with get_user

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Message-id: <200901291541.32618.frank.mehnert@sun.com>
Date:	Thu, 29 Jan 2009 15:41:32 +0100
From:	Frank Mehnert <Frank.Mehnert@....COM>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: PFs on pages pinned with get_user_pages()

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...

Kind regards,

Frank
-- 
Dr.-Ing. Frank Mehnert    Sun Microsystems    http://www.sun.com/

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