fork: clean-up naming of vm_stack/vm_struct variables in vmap stacks code

There are two data types: "struct vm_struct" and "struct vm_stack" that
have the same local variable names: vm_stack, or vm, or s, which makes the
code confusing to read.

Change the code so the naming is consistent:

struct vm_struct is always called vm_area
struct vm_stack is always called vm_stack

One change altering vfree(vm_stack) to vfree(vm_area->addr) may look like
a semantic change but it is not: vm_area->addr points to the vm_stack. 
This was done to improve readability.

[linus.walleij@linaro.org: rebased and added new users of the variable names, address review comments]
Link: https://lore.kernel.org/20240311164638.2015063-4-pasha.tatashin@soleen.com
Link: https://lkml.kernel.org/r/20250509-fork-fixes-v3-2-e6c69dd356f2@linaro.org
Signed-off-by: Pasha Tatashin <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

@@ -198,14 +198,14 @@ struct vm_stack {
 	struct vm_struct *stack_vm_area;
 };
 
-static bool try_release_thread_stack_to_cache(struct vm_struct *vm)
+static bool try_release_thread_stack_to_cache(struct vm_struct *vm_area)
 {
 	unsigned int i;
 
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
 		struct vm_struct *tmp = NULL;
 
-		if (this_cpu_try_cmpxchg(cached_stacks[i], &tmp, vm))
+		if (this_cpu_try_cmpxchg(cached_stacks[i], &tmp, vm_area))
 			return true;
 	}
 	return false;
@@ -214,11 +214,12 @@ static bool try_release_thread_stack_to_cache(struct vm_struct *vm)
 static void thread_stack_free_rcu(struct rcu_head *rh)
 {
 	struct vm_stack *vm_stack = container_of(rh, struct vm_stack, rcu);
+	struct vm_struct *vm_area = vm_stack->stack_vm_area;
 
 	if (try_release_thread_stack_to_cache(vm_stack->stack_vm_area))
 		return;
 
-	vfree(vm_stack);
+	vfree(vm_area->addr);
 }
 
 static void thread_stack_delayed_free(struct task_struct *tsk)
@@ -231,71 +232,68 @@ static void thread_stack_delayed_free(struct task_struct *tsk)
 
 static int free_vm_stack_cache(unsigned int cpu)
 {
-	struct vm_struct **cached_vm_stacks = per_cpu_ptr(cached_stacks, cpu);
+	struct vm_struct **cached_vm_stack_areas = per_cpu_ptr(cached_stacks, cpu);
 	int i;
 
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
-		struct vm_struct *vm_stack = cached_vm_stacks[i];
+		struct vm_struct *vm_area = cached_vm_stack_areas[i];
 
-		if (!vm_stack)
+		if (!vm_area)
 			continue;
 
-		vfree(vm_stack->addr);
-		cached_vm_stacks[i] = NULL;
+		vfree(vm_area->addr);
+		cached_vm_stack_areas[i] = NULL;
 	}
 
 	return 0;
 }
 
-static int memcg_charge_kernel_stack(struct vm_struct *vm)
+static int memcg_charge_kernel_stack(struct vm_struct *vm_area)
 {
 	int i;
 	int ret;
 	int nr_charged = 0;
 
-	BUG_ON(vm->nr_pages != THREAD_SIZE / PAGE_SIZE);
+	BUG_ON(vm_area->nr_pages != THREAD_SIZE / PAGE_SIZE);
 
 	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
-		ret = memcg_kmem_charge_page(vm->pages[i], GFP_KERNEL, 0);
+		ret = memcg_kmem_charge_page(vm_area->pages[i], GFP_KERNEL, 0);
 		if (ret)
 			goto err;
 		nr_charged++;
 	}
 	return 0;
 err:
 	for (i = 0; i < nr_charged; i++)
-		memcg_kmem_uncharge_page(vm->pages[i], 0);
+		memcg_kmem_uncharge_page(vm_area->pages[i], 0);
 	return ret;
 }
 
 static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 {
-	struct vm_struct *vm;
+	struct vm_struct *vm_area;
 	void *stack;
 	int i;
 
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
-		struct vm_struct *s;
-
-		s = this_cpu_xchg(cached_stacks[i], NULL);
-
-		if (!s)
+		vm_area = this_cpu_xchg(cached_stacks[i], NULL);
+		if (!vm_area)
 			continue;
 
 		/* Reset stack metadata. */
-		kasan_unpoison_range(s->addr, THREAD_SIZE);
+		kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
 
-		stack = kasan_reset_tag(s->addr);
+		stack = kasan_reset_tag(vm_area->addr);
 
 		/* Clear stale pointers from reused stack. */
 		memset(stack, 0, THREAD_SIZE);
 
-		if (memcg_charge_kernel_stack(s)) {
-			vfree(s->addr);
+		if (memcg_charge_kernel_stack(vm_area)) {
+			vfree(vm_area->addr);
 			return -ENOMEM;
 		}
 
-		tsk->stack_vm_area = s;
+		tsk->stack_vm_area = vm_area;
 		tsk->stack = stack;
 		return 0;
 	}
@@ -311,8 +309,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 	if (!stack)
 		return -ENOMEM;
 
-	vm = find_vm_area(stack);
-	if (memcg_charge_kernel_stack(vm)) {
+	vm_area = find_vm_area(stack);
+	if (memcg_charge_kernel_stack(vm_area)) {
 		vfree(stack);
 		return -ENOMEM;
 	}
@@ -321,7 +319,7 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 	 * free_thread_stack() can be called in interrupt context,
 	 * so cache the vm_struct.
 	 */
-	tsk->stack_vm_area = vm;
+	tsk->stack_vm_area = vm_area;
 	stack = kasan_reset_tag(stack);
 	tsk->stack = stack;
 	return 0;
@@ -517,11 +515,11 @@ void vm_area_free(struct vm_area_struct *vma)
 static void account_kernel_stack(struct task_struct *tsk, int account)
 {
 	if (IS_ENABLED(CONFIG_VMAP_STACK)) {
-		struct vm_struct *vm = task_stack_vm_area(tsk);
+		struct vm_struct *vm_area = task_stack_vm_area(tsk);
 		int i;
 
 		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
-			mod_lruvec_page_state(vm->pages[i], NR_KERNEL_STACK_KB,
+			mod_lruvec_page_state(vm_area->pages[i], NR_KERNEL_STACK_KB,
 					      account * (PAGE_SIZE / 1024));
 	} else {
 		void *stack = task_stack_page(tsk);
@@ -537,12 +535,12 @@ void exit_task_stack_account(struct task_struct *tsk)
 	account_kernel_stack(tsk, -1);
 
 	if (IS_ENABLED(CONFIG_VMAP_STACK)) {
-		struct vm_struct *vm;
+		struct vm_struct *vm_area;
 		int i;
 
-		vm = task_stack_vm_area(tsk);
+		vm_area = task_stack_vm_area(tsk);
 		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
-			memcg_kmem_uncharge_page(vm->pages[i], 0);
+			memcg_kmem_uncharge_page(vm_area->pages[i], 0);
 	}
 }