From: markj <markj@FreeBSD.org>
Date: Mon, 16 Sep 2019 15:16:48 +0000 (+0000)
Subject: Assert that the refcount value is not VPRC_BLOCKED in vm_page_drop().
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=64ae9dfb9d82a2eb57978aac372f4848736e7f1c;p=freebsd.git

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 07672d38e59..36f8237ab40 100644
--- 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);
 }
 
 /*