xen: disable split pte locks for now

When pinning and unpinning pagetables, we must protect them against
being used by other CPUs, lest they see the pagetable in an
intermediate read-only-but-not-pinned state.

When using split pte locks, doing this properly would require taking
all the pte locks for the pagetable while pinning, but this may overflow
the PREEMPT_BITS part of the preempt counter if the process has mapped
more than about 512M of memory.

However, failing to take the pte locks causes write-protect faults when
the pageout code is trying to clear the Access bit on a pte which is part
of a freshy created and still being pinned process after fork.

This is a short-term fix until the problem is solved properly.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Hugh Dickins <hugh@veritas.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andi Kleen <ak@suse.de>
Cc: Keir Fraser <keir@xensource.com>
Cc: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/Kconfig

@@ -137,6 +137,7 @@ config SPLIT_PTLOCK_CPUS
 	int
 	default "4096" if ARM && !CPU_CACHE_VIPT
 	default "4096" if PARISC && !PA20
+	default "4096" if XEN
 	default "4"
 
 #