Author: alc
Date: Thu Nov 3 16:44:55 2016
New Revision: 308261
URL: https://svnweb.freebsd.org/changeset/base/308261
Log:
In vm_fault()'s loop over the shadow chain, move a comment describing our
invariants to a better place. Also, add two comments concerning the
relationship between the map and vnode locks.
Reviewed by: kib
MFC after: 3 days
Modified:
head/sys/vm/vm_fault.c
Modified: head/sys/vm/vm_fault.c
==============================================================================
--- head/sys/vm/vm_fault.c Thu Nov 3 15:04:16 2016 (r308260)
+++ head/sys/vm/vm_fault.c Thu Nov 3 16:44:55 2016 (r308261)
@@ -564,6 +564,14 @@ fast_failed:
readrest:
/*
+ * At this point, we have either allocated a new page or found
+ * an existing page that is only partially valid.
+ *
+ * We hold a reference on the current object and the page is
+ * exclusive busied.
+ */
+
+ /*
* If the pager for the current object might have the page,
* then determine the number of additional pages to read and
* potentially reprioritize previously read pages for earlier
@@ -632,18 +640,23 @@ readrest:
*/
if (fs.object->type != OBJT_DEFAULT) {
/*
- * We have either allocated a new page or found an
- * existing page that is only partially valid. We
- * hold a reference on fs.object and the page is
- * exclusive busied.
+ * Release the map lock before locking the vnode or
+ * sleeping in the pager. (If the current object has
+ * a shadow, then an earlier iteration of this loop
+ * may have already unlocked the map.)
*/
unlock_map(&fs);
if (fs.object->type == OBJT_VNODE &&
(vp = fs.object->handle) != fs.vp) {
+ /*
+ * Perform an unlock in case the desired vnode
+ * changed while the map was unlocked during a
+ * retry.
+ */
unlock_vp(&fs);
- locked = VOP_ISLOCKED(vp);
+ locked = VOP_ISLOCKED(vp);
if (locked != LK_EXCLUSIVE)
locked = LK_SHARED;
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