Assert that the refcount value is not VPRC_BLOCKED in vm_page_drop().

VPRC_BLOCKED is a refcount flag used to indicate that a thread is
tearing down mappings of a page.  When set, it causes attempts to wire a
page via a pmap lookup to fail.  It should never represent the last
reference to a page, so assert this.

Suggested by:	kib
Reviewed by:	alc, kib
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D21639

diff --git a/sys/vm/vm_page.h b/sys/vm/vm_page.h
index 07672d38e59a4075dd73238f9544b208b64b8981..36f8237ab4077697343dc6e7993c8cf19360021e 100644 (file)
--- a/sys/vm/vm_page.h
+++ b/sys/vm/vm_page.h
@@ -902,13 +902,17 @@ vm_page_in_laundry(vm_page_t m)
 static inline u_int
 vm_page_drop(vm_page_t m, u_int val)
 {
+       u_int old;
 
        /*
         * Synchronize with vm_page_free_prep(): ensure that all updates to the
         * page structure are visible before it is freed.
         */
        atomic_thread_fence_rel();
-       return (atomic_fetchadd_int(&m->ref_count, -val));
+       old = atomic_fetchadd_int(&m->ref_count, -val);
+       KASSERT(old != VPRC_BLOCKED,
+           ("vm_page_drop: page %p has an invalid refcount value", m));
+       return (old);
 }
 
 /*