[PATCH] VM: fix pageout-related deadlock

[Richard Braun]

* vm/vm_map.c (_vm_map_entry_create): Temporarily set the current thread
as VM privileged.
---
 vm/vm_map.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/vm/vm_map.c b/vm/vm_map.c
index 4490878..5d544c4 100644
--- a/vm/vm_map.c
+++ b/vm/vm_map.c
@@ -240,8 +240,26 @@ vm_map_entry_t _vm_map_entry_create(map_header)
        const struct vm_map_header *map_header;
 {
        vm_map_entry_t  entry;
+       boolean_t vm_privilege;
 
+       /*
+        *      XXX Map entry creation may occur while a map is locked,
+        *      for example when clipping entries. If the system is running
+        *      low on memory, allocating an entry may block until pages
+        *      are available. But if a map used by the default pager is
+        *      kept locked, a deadlock occurs.
+        *
+        *      This workaround temporarily elevates the current thread
+        *      VM privileges to avoid that particular deadlock, and does
+        *      so regardless of the map for convenience, and because it's
+        *      currently impossible to predict which map the default pager
+        *      may depend on.
+        */
+       vm_privilege = current_thread()->vm_privilege;
+       current_thread()->vm_privilege = TRUE;
        entry = (vm_map_entry_t) kmem_cache_alloc(&vm_map_entry_cache);
+       current_thread()->vm_privilege = vm_privilege;
+
        if (entry == VM_MAP_ENTRY_NULL)
                panic("vm_map_entry_create");
 
-- 
2.6.4