svn commit: r251813 - user/attilio/vmobj-readlock/sys/vm

[Attilio Rao]

Author: attilio
Date: Sun Jun 16 18:31:12 2013
New Revision: 251813
URL: http://svnweb.freebsd.org/changeset/base/251813

Log:
  Tentatively document the hard/soft busy logic.
  
  Sponsored by:	EMC / Isilon storage division

Modified:
  user/attilio/vmobj-readlock/sys/vm/vm_page.h

Modified: user/attilio/vmobj-readlock/sys/vm/vm_page.h
==============================================================================
--- user/attilio/vmobj-readlock/sys/vm/vm_page.h	Sun Jun 16 18:27:53 2013	(r251812)
+++ user/attilio/vmobj-readlock/sys/vm/vm_page.h	Sun Jun 16 18:31:12 2013	(r251813)
@@ -109,6 +109,17 @@
  *	contains the dirty field.  In the machine-independent layer,
  *	the implementation of read-modify-write operations on the
  *	field is encapsulated in vm_page_clear_dirty_mask().
+ *
+ *	Page content access is regulated (mostly) by the busy mechanism.
+ *	When the page content is changing (for example, during a page READ
+ *	operation) the page owner must acquire an hard busy token. Similarly,
+ *	when the page content is just being accessed for reading purposes
+ *	(for example, during a page WRITE operation) the page owner must
+ *	acquire a soft busy token.
+ *	The hard busy mechanism is controlled using vm_page_busy() and
+ *	vm_page_unbusy() interfaces.  Likewise the soft busy mechanism is
+ *	controlled through the usage of vm_page_io_start() and
+ *	vm_page_io_finish().
  */
 
 #if PAGE_SIZE == 4096