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)

1 files changed, 11 insertions, 4 deletions

diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index 50bf725..5e81333 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -292,7 +292,7 @@ vm_fault_hold(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
 	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;
 		}
 
 		/*