Author: alc
Date: Wed Jul 20 17:20:22 2016
New Revision: 303101
URL: https://svnweb.freebsd.org/changeset/base/303101
Log:
Add a comment describing the 'fast path' that was introduced in r270011.
Reviewed by: kib
MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division
Modified:
head/sys/vm/vm_fault.c
Modified: head/sys/vm/vm_fault.c
==============================================================================
--- head/sys/vm/vm_fault.c Wed Jul 20 17:19:47 2016 (r303100)
+++ head/sys/vm/vm_fault.c Wed Jul 20 17:20:22 2016 (r303101)
@@ -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() */
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