[PATCH v2 3/5] fork: Remove assumption that vm_area->nr_pages equals to THREAD_SIZE

[Linus Walleij <linus.walleij@linaro.org>]

From: Pasha Tatashin <pasha.tatashin@soleen.com>

In many places number of pages in the stack is detremined via
(THREAD_SIZE / PAGE_SIZE). There is also a BUG_ON() that ensures that
(THREAD_SIZE / PAGE_SIZE) is indeed equal to vm_area->nr_pages.

Consistently use vm_area->nr_pages to determine the actual number
of pages allocated in the stack instead, so it is clear what is
going on here.

The assignment of a local variable nr_pages in
memcg_charge_kernel_stack() takes roughly the same amount of time
as the BUG() assertion, and the two other sites arguably should
have had a BUG() assertion as well.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/20240311164638.2015063-5-pasha.tatashin@soleen.com
[linus.walleij@linaro.orh: Rebased, initialized helpers variables in declaration]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 kernel/fork.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 8b8457562740c114c640a8cc230876f6a286b246..c60a0ec61a421324e3733b506d99531c5965cbc6 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -253,10 +253,9 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm_area)
 	int i;
 	int ret;
 	int nr_charged = 0;
+	int nr_pages = vm_area->nr_pages;
 
-	BUG_ON(vm_area->nr_pages != THREAD_SIZE / PAGE_SIZE);
-
-	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
+	for (i = 0; i < nr_pages; i++) {
 		ret = memcg_kmem_charge_page(vm_area->pages[i], GFP_KERNEL, 0);
 		if (ret)
 			goto err;
@@ -516,9 +515,10 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 {
 	if (IS_ENABLED(CONFIG_VMAP_STACK)) {
 		struct vm_struct *vm_area = task_stack_vm_area(tsk);
+		int nr_pages = vm_area->nr_pages;
 		int i;
 
-		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+		for (i = 0; i < nr_pages; i++)
 			mod_lruvec_page_state(vm_area->pages[i], NR_KERNEL_STACK_KB,
 					      account * (PAGE_SIZE / 1024));
 	} else {
@@ -535,11 +535,11 @@ void exit_task_stack_account(struct task_struct *tsk)
 	account_kernel_stack(tsk, -1);
 
 	if (IS_ENABLED(CONFIG_VMAP_STACK)) {
-		struct vm_struct *vm_area;
+		struct vm_struct *vm_area = task_stack_vm_area(tsk);
+		int nr_pages = vm_area->nr_pages;
 		int i;
 
-		vm_area = task_stack_vm_area(tsk);
-		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+		for (i = 0; i < nr_pages; i++)
 			memcg_kmem_uncharge_page(vm_area->pages[i], 0);
 	}
 }

-- 
2.49.0