svn commit: r302236 - head/sys/vm

[Konstantin Belousov]

Author: kib
Date: Mon Jun 27 21:54:19 2016
New Revision: 302236
URL: https://svnweb.freebsd.org/changeset/base/302236

Log:
  If the vm_fault() handler raced with the vm_object_collapse()
  sleepable scan, iteration over the shadow chain looking for a page
  could find an OBJ_DEAD object.  Such state of the mapping is only
  transient, the dead object will be terminated and removed from the
  chain shortly.  We must not return KERN_PROTECTION_FAILURE unless the
  object type is changed to OBJT_DEAD in the chain, indicating that
  paging on this address is really impossible.  Returning
  KERN_PROTECTION_FAILURE prematurely causes spurious SIGSEGV delivered
  to processes, or kernel accesses to UVA spuriously failing with
  EFAULT.
  
  If the object with OBJ_DEAD flag is found, only return
  KERN_PROTECTION_FAILURE when object type is already OBJT_DEAD.
  Otherwise, sleep a tick and retry the fault handling.
  
  Ideally, we would wait until the OBJ_DEAD flag is resolved, e.g. by
  waiting until the paging on this object is finished.  But to do so, we
  need to reference the dead object, while vm_object_collapse() insists
  on owning the final reference on the collapsed object.  This could be
  fixed by e.g. changing the assert to shared reference release between
  vm_fault() and vm_object_collapse(), but it seems to be too much
  complications for rare boundary condition.
  
  PR:	204426
  Tested by:    pho
  Reviewed by:  alc
  Sponsored by: The FreeBSD Foundation
  X-Differential revision:	https://reviews.freebsd.org/D6085
  MFC after:	2 weeks
  Approved by:	re (gjb)

Modified:
  head/sys/vm/vm_fault.c

Modified: head/sys/vm/vm_fault.c
==============================================================================
--- head/sys/vm/vm_fault.c	Mon Jun 27 21:52:17 2016	(r302235)
+++ head/sys/vm/vm_fault.c	Mon Jun 27 21:54:19 2016	(r302236)
@@ -292,7 +292,7 @@ vm_fault_hold(vm_map_t map, vm_offset_t 
 	struct faultstate fs;
 	struct vnode *vp;
 	vm_page_t m;
-	int ahead, behind, cluster_offset, error, locked;
+	int ahead, behind, cluster_offset, dead, error, locked;
 
 	hardfault = 0;
 	growstack = TRUE;
@@ -421,11 +421,18 @@ fast_failed:
 	fs.pindex = fs.first_pindex;
 	while (TRUE) {
 		/*
-		 * If the object is dead, we stop here
+		 * If the object is marked for imminent termination,
+		 * we retry here, since the collapse pass has raced
+		 * with us.  Otherwise, if we see terminally dead
+		 * object, return fail.
 		 */
-		if (fs.object->flags & OBJ_DEAD) {
+		if ((fs.object->flags & OBJ_DEAD) != 0) {
+			dead = fs.object->type == OBJT_DEAD;
 			unlock_and_deallocate(&fs);
-			return (KERN_PROTECTION_FAILURE);
+			if (dead)
+				return (KERN_PROTECTION_FAILURE);
+			pause("vmf_de", 1);
+			goto RetryFault;
 		}
 
 		/*