arm kernel oops in highmem.c with 4.2

[msalter@redhat.com (Mark Salter)]

On Tue, 2015-08-11 at 22:18 -0400, Nicolas Pitre wrote:
> On Tue, 11 Aug 2015, Mark Salter wrote:
> 
> > On Tue, 2015-08-11 at 16:20 -0400, Nicolas Pitre wrote:
> > > On Tue, 11 Aug 2015, Russell King - ARM Linux wrote:
> > > 
> > > > It needs fixing or removing.
> > > 
> > > Absolutely. I'm not disputing that. I'm only asking so we can wisely 
> > > choose between fixing or removing.  Personally I'd be inclined 
> > > towards 
> > > the later, unless the following is sufficient to fix it:
> > > 
> > > diff --git a/arch/arm/lib/uaccess_with_memcpy.c 
> > > b/arch/arm/lib/uaccess_with_memcpy.c
> > > index 3e58d71001..4b39af2dfd 100644
> > > --- a/arch/arm/lib/uaccess_with_memcpy.c
> > > +++ b/arch/arm/lib/uaccess_with_memcpy.c
> > > @@ -96,7 +96,7 @@ __copy_to_user_memcpy(void __user *to, const void 
> > > *from, unsigned long n)
> > >  	}
> > >  
> > >  	/* the mmap semaphore is taken only if not in an atomic 
> > > context 
> > > */
> > > -	atomic = in_atomic();
> > > +	atomic = faulthandler_disabled();
> > >  
> > >  	if (!atomic)
> > >  		down_read(&current->mm->mmap_sem);
> > 
> > Yeah, that fixes the problem I was seeing.
> 
> Good!  Then I'll add it to RMK's patch system.  May I add a tested-by 
> tag with your name?

Yes

> 
> > This was on a calxeda highbank.
> 
> Any idea why this option was set?

None. It seems to have been there in the original fedora config
for v7 kernels. But nothing in the config turns on CPU_FEROCEON
so there is no real need for it.

> 
> > With CONFIG_UACCESS_WITH_MEMCPY, the dd copy test reported 1.8GB/s
> > Without CONFIG_UACCESS_WITH_MEMCPY, 2.1GB/s
> 
> Therefore this is of no benefit to you.
> 
> 
> Nicolas