svn commit: r303101 - head/sys/vm

[Alan Cox]

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() */