xen: lock pte pages while pinning/unpinning

When a pagetable is created, it is made globally visible in the rmap
prio tree before it is pinned via arch_dup_mmap(), and remains in the
rmap tree while it is unpinned with arch_exit_mmap().

This means that other CPUs may race with the pinning/unpinning
process, and see a pte between when it gets marked RO and actually
pinned, causing any pte updates to fail with write-protect faults.

As a result, all pte pages must be properly locked, and only unlocked
once the pinning/unpinning process has finished.

In order to avoid taking spinlocks for the whole pagetable - which may
overflow the PREEMPT_BITS portion of preempt counter - it locks and pins
each pte page individually, and then finally pins the whole pagetable.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickens <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>

1 files changed, 0 insertions, 1 deletions

diff --git a/mm/Kconfig b/mm/Kconfig
index a7609cbcb00d..e24d348083c3 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -137,7 +137,6 @@ config SPLIT_PTLOCK_CPUS
         int
         default "4096" if ARM && !CPU_CACHE_VIPT
         default "4096" if PARISC && !PA20
-        default "4096" if XEN
         default "4"
 
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