swap-like pagers: assert that writemapping decrease does not pass zero

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D45119

Author: kostikbel

sys/fs/tmpfs/tmpfs_subr.c

@@ -183,6 +183,9 @@ tmpfs_pager_release_writecount(vm_object_t object, vm_offset_t start,
 	KASSERT((object->flags & OBJ_ANON) == 0,
 	    ("%s: object %p with OBJ_ANON", __func__, object));
 	old = object->un_pager.swp.writemappings;
+	KASSERT(old >= (vm_ooffset_t)end - start,
+	    ("tmpfs obj %p writecount %jx dec %jx", object, (uintmax_t)old,
+	    (uintmax_t)((vm_ooffset_t)end - start)));
 	object->un_pager.swp.writemappings -= (vm_ooffset_t)end - start;
 	new = object->un_pager.swp.writemappings;
 	tmpfs_pager_writecount_recalc(object, old, new);

sys/vm/swap_pager.c

@@ -3158,6 +3158,10 @@ swap_pager_release_writecount(vm_object_t object, vm_offset_t start,
 	VM_OBJECT_WLOCK(object);
 	KASSERT((object->flags & OBJ_ANON) == 0,
 	    ("Splittable object with writecount"));
+	KASSERT(object->un_pager.swp.writemappings >= (vm_ooffset_t)end - start,
+	    ("swap obj %p writecount %jx dec %jx", object,
+	    (uintmax_t)object->un_pager.swp.writemappings,
+	    (uintmax_t)((vm_ooffset_t)end - start)));
 	object->un_pager.swp.writemappings -= (vm_ooffset_t)end - start;
 	VM_OBJECT_WUNLOCK(object);
 }