Author: attilio
Date: Fri Jun 22 18:34:11 2012
New Revision: 237451
URL: http://svn.freebsd.org/changeset/base/237451
Log:
- Add a comment explaining the locking of the cached pages pool held
by vm_objects.
- Add flags for the per-object lock and free pages queue mutex lock.
Use the newly added flags to mark the cache root within the vm_object
structure.
Please note that other vm_object members should be marked with correct
locking but they are left for other commits.
In collabouration with: alc
MFC after: 3 days3 days3 days
Modified:
head/sys/vm/vm_object.h
Modified: head/sys/vm/vm_object.h
==============================================================================
--- head/sys/vm/vm_object.h Fri Jun 22 18:20:26 2012 (r237450)
+++ head/sys/vm/vm_object.h Fri Jun 22 18:34:11 2012 (r237451)
@@ -76,8 +76,21 @@
*
* vm_object_t Virtual memory object.
*
+ * The root of cached pages pool is protected by both the per-object mutex
+ * and the free pages queue mutex.
+ * On insert in the cache splay tree, the per-object mutex is expected
+ * to be already held and the free pages queue mutex will be
+ * acquired during the operation too.
+ * On remove and lookup from the cache splay tree, only the free
+ * pages queue mutex is expected to be locked.
+ * These rules allow for reliably checking for the presence of cached
+ * pages with only the per-object lock held, thereby reducing contention
+ * for the free pages queue mutex.
+ *
* List of locks
* (c) const until freed
+ * (o) per-object mutex
+ * (f) free pages queue mutex
*
*/
@@ -102,7 +115,7 @@ struct vm_object {
vm_ooffset_t backing_object_offset;/* Offset in backing object */
TAILQ_ENTRY(vm_object) pager_object_list; /* list of all objects of
this pager type */
LIST_HEAD(, vm_reserv) rvq; /* list of reservations */
- vm_page_t cache; /* root of the cache page splay tree */
+ vm_page_t cache; /* (o + f) root of the cache page splay
tree */
void *handle;
union {
/*
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