svn commit: r265948 - head/sys/vm

Alan Cox alc at FreeBSD.org
Tue May 13 13:20:23 UTC 2014


Author: alc
Date: Tue May 13 13:20:23 2014
New Revision: 265948
URL: http://svnweb.freebsd.org/changeset/base/265948

Log:
  On a fork allow read-only wired pages to be copy-on-write shared between the
  parent and child processes.  Previously, we copied these pages even though
  they are read only.  However, the reason for copying them is historical and
  no longer exists.  In recent times, vm_map_protect() has developed the
  ability to copy pages when write access is added to wired copy-on-write
  pages.  So, in this case, copy-on-write sharing of wired pages is not to be
  feared.  It is not going to lead to copy-on-write faults on wired memory.
  
  Reviewed by:	kib
  MFC after:	1 week
  Sponsored by:	EMC / Isilon Storage Division

Modified:
  head/sys/vm/vm_map.c

Modified: head/sys/vm/vm_map.c
==============================================================================
--- head/sys/vm/vm_map.c	Tue May 13 06:09:01 2014	(r265947)
+++ head/sys/vm/vm_map.c	Tue May 13 13:20:23 2014	(r265948)
@@ -3024,8 +3024,8 @@ vm_map_copy_entry(
 	if ((dst_entry->eflags|src_entry->eflags) & MAP_ENTRY_IS_SUB_MAP)
 		return;
 
-	if (src_entry->wired_count == 0) {
-
+	if (src_entry->wired_count == 0 ||
+	    (src_entry->protection & VM_PROT_WRITE) == 0) {
 		/*
 		 * If the source entry is marked needs_copy, it is already
 		 * write-protected.
@@ -3116,9 +3116,9 @@ vm_map_copy_entry(
 		    dst_entry->end - dst_entry->start, src_entry->start);
 	} else {
 		/*
-		 * Of course, wired down pages can't be set copy-on-write.
-		 * Cause wired pages to be copied into the new map by
-		 * simulating faults (the new pages are pageable)
+		 * We don't want to make writeable wired pages copy-on-write.
+		 * Immediately copy these pages into the new map by simulating
+		 * page faults.  The new pages are pageable.
 		 */
 		vm_fault_copy_entry(dst_map, src_map, dst_entry, src_entry,
 		    fork_charge);


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