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From: Alan Cox <alc@FreeBSD.org>
Date: Sat, 23 Jul 2016 21:56:52 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r303251 - stable/11/sys/vm
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X-List-Received-Date: Sat, 23 Jul 2016 21:56:53 -0000
Author: alc
Date: Sat Jul 23 21:56:52 2016
New Revision: 303251
URL: https://svnweb.freebsd.org/changeset/base/303251
Log:
MFC r303101
Add a comment describing the 'fast path' that was introduced in r270011.
Approved by: re (gjb)
Modified:
stable/11/sys/vm/vm_fault.c
Directory Properties:
stable/11/ (props changed)
Modified: stable/11/sys/vm/vm_fault.c
==============================================================================
--- stable/11/sys/vm/vm_fault.c Sat Jul 23 21:22:26 2016 (r303250)
+++ stable/11/sys/vm/vm_fault.c Sat Jul 23 21:56:52 2016 (r303251)
@@ -354,6 +354,15 @@ RetryFault:;
KASSERT((fault_flags & VM_FAULT_WIRE) == 0,
("!wired && VM_FAULT_WIRE"));
+ /*
+ * Try to avoid lock contention on the top-level object through
+ * special-case handling of some types of page faults, specifically,
+ * those that are both (1) mapping an existing page from the top-
+ * level object and (2) not having to mark that object as containing
+ * dirty pages. Under these conditions, a read lock on the top-level
+ * object suffices, allowing multiple page faults of a similar type to
+ * run in parallel on the same top-level object.
+ */
if (fs.vp == NULL /* avoid locked vnode leak */ &&
(fault_flags & (VM_FAULT_WIRE | VM_FAULT_DIRTY)) == 0 &&
/* avoid calling vm_object_set_writeable_dirty() */