rfork(2): fix swap accounting in vmspace_unshare()

When an attempt to increase the swap charge for the ucred failed, we
must forcibly increase the charge to allow the vmspace_destroy()
operation to correctly un-charge the accumulated objects.

Add a swap_reserve_force_by_cred() helper and use it in
vmspace_unshare(), same as it is done in normal fork operations.

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D54572

sys/vm/swap_pager.c

@@ -330,7 +330,7 @@ swap_reserve_by_cred(vm_ooffset_t incr, struct ucred *cred)
 }
 
 void
-swap_reserve_force(vm_ooffset_t incr)
+swap_reserve_force_by_cred(vm_ooffset_t incr, struct ucred *cred)
 {
 	u_long pincr;
 
@@ -346,7 +346,13 @@ swap_reserve_force(vm_ooffset_t incr)
 #endif
 	pincr = atop(incr);
 	atomic_add_long(&swap_reserved, pincr);
-	swap_reserve_force_rlimit(pincr, curthread->td_ucred);
+	swap_reserve_force_rlimit(pincr, cred);
+}
+
+void
+swap_reserve_force(vm_ooffset_t incr)
+{
+	swap_reserve_force_by_cred(incr, curthread->td_ucred);
 }
 
 void

sys/vm/vm.h

@@ -168,6 +168,7 @@ void vm_ksubmap_init(struct kva_md_info *);
 bool swap_reserve(vm_ooffset_t incr);
 bool swap_reserve_by_cred(vm_ooffset_t incr, struct ucred *cred);
 void swap_reserve_force(vm_ooffset_t incr);
+void swap_reserve_force_by_cred(vm_ooffset_t incr, struct ucred *cred);
 void swap_release(vm_ooffset_t decr);
 void swap_release_by_cred(vm_ooffset_t decr, struct ucred *cred);
 

sys/vm/vm_map.c

@@ -4957,6 +4957,13 @@ vmspace_unshare(struct proc *p)
 	if (newvmspace == NULL)
 		return (ENOMEM);
 	if (!swap_reserve_by_cred(fork_charge, p->p_ucred)) {
+		/*
+		 * The swap reservation failed. The accounting from
+		 * the entries of the copied newvmspace will be
+		 * subtracted in vmspace_free(), so force the
+		 * reservation there.
+		 */
+		swap_reserve_force_by_cred(fork_charge, p->p_ucred);
 		vmspace_free(newvmspace);
 		return (ENOMEM);
 	}