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 <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Keir Fraser <[email protected]>
Cc: Jan Beulich <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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"
 
 #