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

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Sep 28, 2009 at 01:16:25PM +0300, Imre Deak wrote:
> 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.

Now you're talking about different code - the bit I quoted was what was
in your submitted patch, without deletion of intervening lines.  There
was no else clause in your patch.

Please, go back and look at your original patch.