arm kernel oops in highmem.c with 4.2

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

On Tue, Aug 11, 2015 at 03:41:52PM -0400, Nicolas Pitre wrote:
> I'd agree.  But first I'd like to know why the fedora kernel is using 
> CONFIG_UACCESS_WITH_MEMCPY?  If it's just because it sounded cool then 
> that's a bad reason.
> 
> That code was created to work around inneficiency in the Orion CPU core 
> that didn't coalesce writes from STRT instructions, and by using plain 
> STR and/or STM the actual throughput was more than doubled.  This was 
> fixed in later cores. However Orion users (if they still exist) might 
> like the added performance. I don't have Orion-based targets anymore 
> (they took the way of the recycling facility a while ago).

Irrespective of that, what has been found is that the implementation is
unsafe - it is taking a semaphore when page faults are disabled.  In
other words, notwithstanding the above, it's buggy no matter if it's
an Orion CPU core, or highmem, or what.  Any place in the kernel which
uses pagefault_disable() and then calls into this code will be buggy.

It needs fixing or removing.

Even if it is fixed, making it _depend_ on CPU_FEROCEON sounds like a
good idea if non-orion stuff isn't supposed to be enabling it.  Or
something like that.

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