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) {
                        /*
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