MFC r303101

  Add a comment describing the 'fast path' that was introduced in r270011.

1 files changed, 9 insertions, 0 deletions

diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index d65cda2..909f57f 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -353,6 +353,15 @@ 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() */