[PATCH] ARM: add warning for invalid kernel page faults

[imre.deak@nokia.com (Imre Deak)]

On Mon, Sep 28, 2009 at 12:04:42PM +0200, ext Russell King - ARM Linux wrote:
> On Mon, Sep 28, 2009 at 01:00:48PM +0300, Imre Deak wrote:
> > On Mon, Sep 28, 2009 at 11:55:16AM +0200, ext Russell King - ARM Linux wrote:
> > > On Mon, Sep 28, 2009 at 12:48:24PM +0300, Imre Deak wrote:
> > > > To easier detect code that can trigger the above error, add a check
> > > > also for the case where mmap_sem is acquired. As this has an overhead
> > > > make it a VM debug warning.
> > > 
> > > It _is_ already easy.  I'm not sure why you want even more noise, and
> > > why you want to break the page fault handling.  From the warning you
> > > received in your previous post, it said:
> > 
> > The problem is that it happens very rarely. Only if at the time of the
> > fault the mmap_sem happens to be held. With the change the error would
> > be apparent at the first fault the offending instruction generates.
> 
> Actually... I don't agree that your code can have any change what so
> ever.
> 
> Condition 1:
>                 if (!user_mode(regs) && !search_exception_tables(regs->ARM_pc))
>                         goto no_context;
> 
>                 down_read(&mm->mmap_sem);
> +#ifdef CONFIG_DEBUG_VM
> 
> Condition 2:
> 
> +               if (!user_mode(regs) &&
> +                   !search_exception_tables(regs->ARM_pc)) {
> +                       static unsigned long last_warn_jiffies;
> 
> Condition 1 and condition 2 are identical.  They do not change on whether
> the mmap_sem is held or not.  So please explain to me how you actually
> get to printing any of your new warnings.

With the change it's:

Condition 1:

if (!down_read_trylock(&mm->mmap_sem)) {
	if (!user_mode(regs) && !search_exception_tables(regs->ARM_pc))
		goto no_context;
	down_read(&mm->mmap_sem);
} else {
#ifdef CONFIG_DEBUG_VM

Condition 2:
	if (!user_mode(regs) && !search_exception_tables(regs->ARM_pc))


So in condition 2, we managed to acquire mmap_sem, which will be the
majority of the cases for kernel mode faults. In condition 1, we didn't
since it was held by another thread.

--Imre