svn commit: r242434 - head/sys/vm

[Alan Cox]

Author: alc
Date: Thu Nov  1 16:20:02 2012
New Revision: 242434
URL: http://svn.freebsd.org/changeset/base/242434

Log:
  In general, we call pmap_remove_all() before calling vm_page_cache().  So,
  the call to pmap_remove_all() within vm_page_cache() is usually redundant.
  This change eliminates that call to pmap_remove_all() and introduces a
  call to pmap_remove_all() before vm_page_cache() in the one place where
  it didn't already exist.
  
  When iterating over a paging queue, if the object containing the current
  page has a zero reference count, then the page can't have any managed
  mappings.  So, a call to pmap_remove_all() is pointless.
  
  Change a panic() call in vm_page_cache() to a KASSERT().
  
  MFC after:	6 weeks

Modified:
  head/sys/vm/vm_page.c
  head/sys/vm/vm_pageout.c

Modified: head/sys/vm/vm_page.c
==============================================================================
--- head/sys/vm/vm_page.c	Thu Nov  1 15:17:43 2012	(r242433)
+++ head/sys/vm/vm_page.c	Thu Nov  1 16:20:02 2012	(r242434)
@@ -2277,9 +2277,9 @@ vm_page_cache(vm_page_t m)
 	if ((m->oflags & (VPO_UNMANAGED | VPO_BUSY)) || m->busy ||
 	    m->hold_count || m->wire_count)
 		panic("vm_page_cache: attempting to cache busy page");
-	pmap_remove_all(m);
-	if (m->dirty != 0)
-		panic("vm_page_cache: page %p is dirty", m);
+	KASSERT(!pmap_page_is_mapped(m),
+	    ("vm_page_cache: page %p is mapped", m));
+	KASSERT(m->dirty == 0, ("vm_page_cache: page %p is dirty", m));
 	if (m->valid == 0 || object->type == OBJT_DEFAULT ||
 	    (object->type == OBJT_SWAP &&
 	    !vm_pager_has_page(object, m->pindex, NULL, NULL))) {

Modified: head/sys/vm/vm_pageout.c
==============================================================================
--- head/sys/vm/vm_pageout.c	Thu Nov  1 15:17:43 2012	(r242433)
+++ head/sys/vm/vm_pageout.c	Thu Nov  1 16:20:02 2012	(r242434)
@@ -594,7 +594,7 @@ vm_pageout_launder(int queue, int tries,
 			continue;
 		}
 		vm_page_test_dirty(m);
-		if (m->dirty == 0)
+		if (m->dirty == 0 && object->ref_count != 0)
 			pmap_remove_all(m);
 		if (m->dirty != 0) {
 			vm_page_unlock(m);
@@ -1059,31 +1059,16 @@ vm_pageout_scan(int pass)
 		}
 
 		/*
-		 * If the upper level VM system does not believe that the page
-		 * is fully dirty, but it is mapped for write access, then we
-		 * consult the pmap to see if the page's dirty status should
-		 * be updated.
+		 * If the page appears to be clean at the machine-independent
+		 * layer, then remove all of its mappings from the pmap in
+		 * anticipation of placing it onto the cache queue.  If,
+		 * however, any of the page's mappings allow write access,
+		 * then the page may still be modified until the last of those
+		 * mappings are removed.
 		 */
-		if (m->dirty != VM_PAGE_BITS_ALL &&
-		    pmap_page_is_write_mapped(m)) {
-			/*
-			 * Avoid a race condition: Unless write access is
-			 * removed from the page, another processor could
-			 * modify it before all access is removed by the call
-			 * to vm_page_cache() below.  If vm_page_cache() finds
-			 * that the page has been modified when it removes all
-			 * access, it panics because it cannot cache dirty
-			 * pages.  In principle, we could eliminate just write
-			 * access here rather than all access.  In the expected
-			 * case, when there are no last instant modifications
-			 * to the page, removing all access will be cheaper
-			 * overall.
-			 */
-			if (pmap_is_modified(m))
-				vm_page_dirty(m);
-			else if (m->dirty == 0)
-				pmap_remove_all(m);
-		}
+		vm_page_test_dirty(m);
+		if (m->dirty == 0 && object->ref_count != 0)
+			pmap_remove_all(m);
 
 		if (m->valid == 0) {
 			/*