fork: check charging success before zeroing stack

No need to do zero cached stack if memcg charge fails, so move the
charging attempt before the memset operation.

[linus.walleij@linaro.org: rebased]
Link: https://lkml.kernel.org/r/20250509-fork-fixes-v3-3-e6c69dd356f2@linaro.org
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/20240311164638.2015063-6-pasha.tatashin@soleen.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: soleen

kernel/fork.c

@@ -280,6 +280,11 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		if (!vm_area)
 			continue;
 
+		if (memcg_charge_kernel_stack(vm_area)) {
+			vfree(vm_area->addr);
+			return -ENOMEM;
+		}
+
 		/* Reset stack metadata. */
 		kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
 
@@ -288,11 +293,6 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		/* Clear stale pointers from reused stack. */
 		memset(stack, 0, THREAD_SIZE);
 
-		if (memcg_charge_kernel_stack(vm_area)) {
-			vfree(vm_area->addr);
-			return -ENOMEM;
-		}
-
 		tsk->stack_vm_area = vm_area;
 		tsk->stack = stack;
 		return 0;