svn commit: r272907 - head/sys/vm

[Konstantin Belousov]

Author: kib
Date: Fri Oct 10 19:27:36 2014
New Revision: 272907
URL: https://svnweb.freebsd.org/changeset/base/272907

Log:
  Make MAP_NOSYNC handling in the vm_fault() read-locked object path
  compatible with write-locked path.  Test for MAP_ENTRY_NOSYNC and set
  VPO_NOSYNC for pages with dirty mask zero (this does not exclude a
  possibility that the page is dirty, e.g. due to read fault on
  writeable mapping and consequent write; the same issue exists in the
  slow path).
  
  Use helper vm_fault_dirty() to unify fast and slow path handling of
  VPO_NOSYNC and setting the dirty mask.
  
  Reviewed by:	alc
  Sponsored by:	The FreeBSD Foundation

Modified:
  head/sys/vm/vm_fault.c

Modified: head/sys/vm/vm_fault.c
==============================================================================
--- head/sys/vm/vm_fault.c	Fri Oct 10 19:26:26 2014	(r272906)
+++ head/sys/vm/vm_fault.c	Fri Oct 10 19:27:36 2014	(r272907)
@@ -174,6 +174,70 @@ unlock_and_deallocate(struct faultstate 
 	}
 }
 
+static void
+vm_fault_dirty(vm_map_entry_t entry, vm_page_t m, vm_prot_t prot,
+    vm_prot_t fault_type, int fault_flags, boolean_t set_wd)
+{
+	boolean_t need_dirty;
+
+	if (((prot & VM_PROT_WRITE) == 0 &&
+	    (fault_flags & VM_FAULT_DIRTY) == 0) ||
+	    (m->oflags & VPO_UNMANAGED) != 0)
+		return;
+
+	VM_OBJECT_ASSERT_LOCKED(m->object);
+
+	need_dirty = ((fault_type & VM_PROT_WRITE) != 0 &&
+	    (fault_flags & VM_FAULT_CHANGE_WIRING) == 0) ||
+	    (fault_flags & VM_FAULT_DIRTY) != 0;
+
+	if (set_wd)
+		vm_object_set_writeable_dirty(m->object);
+	else
+		/*
+		 * If two callers of vm_fault_dirty() with set_wd ==
+		 * FALSE, one for the map entry with MAP_ENTRY_NOSYNC
+		 * flag set, other with flag clear, race, it is
+		 * possible for the no-NOSYNC thread to see m->dirty
+		 * != 0 and not clear VPO_NOSYNC.  Take vm_page lock
+		 * around manipulation of VPO_NOSYNC and
+		 * vm_page_dirty() call, to avoid the race and keep
+		 * m->oflags consistent.
+		 */
+		vm_page_lock(m);
+
+	/*
+	 * If this is a NOSYNC mmap we do not want to set VPO_NOSYNC
+	 * if the page is already dirty to prevent data written with
+	 * the expectation of being synced from not being synced.
+	 * Likewise if this entry does not request NOSYNC then make
+	 * sure the page isn't marked NOSYNC.  Applications sharing
+	 * data should use the same flags to avoid ping ponging.
+	 */
+	if ((entry->eflags & MAP_ENTRY_NOSYNC) != 0) {
+		if (m->dirty == 0) {
+			m->oflags |= VPO_NOSYNC;
+		}
+	} else {
+		m->oflags &= ~VPO_NOSYNC;
+	}
+
+	/*
+	 * If the fault is a write, we know that this page is being
+	 * written NOW so dirty it explicitly to save on
+	 * pmap_is_modified() calls later.
+	 *
+	 * Also tell the backing pager, if any, that it should remove
+	 * any swap backing since the page is now dirty.
+	 */
+	if (need_dirty)
+		vm_page_dirty(m);
+	if (!set_wd)
+		vm_page_unlock(m);
+	if (need_dirty)
+		vm_pager_page_unswapped(m);
+}
+
 /*
  * TRYPAGER - used by vm_fault to calculate whether the pager for the
  *	      current object *might* contain the page.
@@ -321,11 +385,8 @@ RetryFault:;
 			vm_page_hold(m);
 			vm_page_unlock(m);
 		}
-		if ((fault_type & VM_PROT_WRITE) != 0 &&
-		    (m->oflags & VPO_UNMANAGED) == 0) {
-			vm_page_dirty(m);
-			vm_pager_page_unswapped(m);
-		}
+		vm_fault_dirty(fs.entry, m, prot, fault_type, fault_flags,
+		    FALSE);
 		VM_OBJECT_RUNLOCK(fs.first_object);
 		if (!wired)
 			vm_fault_prefault(&fs, vaddr, 0, 0);
@@ -898,42 +959,7 @@ vnode_locked:
 	if (hardfault)
 		fs.entry->next_read = fs.pindex + faultcount - reqpage;
 
-	if (((prot & VM_PROT_WRITE) != 0 ||
-	    (fault_flags & VM_FAULT_DIRTY) != 0) &&
-	    (fs.m->oflags & VPO_UNMANAGED) == 0) {
-		vm_object_set_writeable_dirty(fs.object);
-
-		/*
-		 * If this is a NOSYNC mmap we do not want to set VPO_NOSYNC
-		 * if the page is already dirty to prevent data written with
-		 * the expectation of being synced from not being synced.
-		 * Likewise if this entry does not request NOSYNC then make
-		 * sure the page isn't marked NOSYNC.  Applications sharing
-		 * data should use the same flags to avoid ping ponging.
-		 */
-		if (fs.entry->eflags & MAP_ENTRY_NOSYNC) {
-			if (fs.m->dirty == 0)
-				fs.m->oflags |= VPO_NOSYNC;
-		} else {
-			fs.m->oflags &= ~VPO_NOSYNC;
-		}
-
-		/*
-		 * If the fault is a write, we know that this page is being
-		 * written NOW so dirty it explicitly to save on 
-		 * pmap_is_modified() calls later.
-		 *
-		 * Also tell the backing pager, if any, that it should remove
-		 * any swap backing since the page is now dirty.
-		 */
-		if (((fault_type & VM_PROT_WRITE) != 0 &&
-		    (fault_flags & VM_FAULT_CHANGE_WIRING) == 0) ||
-		    (fault_flags & VM_FAULT_DIRTY) != 0) {
-			vm_page_dirty(fs.m);
-			vm_pager_page_unswapped(fs.m);
-		}
-	}
-
+	vm_fault_dirty(fs.entry, fs.m, prot, fault_type, fault_flags, TRUE);
 	vm_page_assert_xbusied(fs.m);
 
 	/*