[RFC PATCH 01/15] fork: Remove assumption that vm_area->nr_pages equals to THREAD_SIZE

[Mostafa Saleh]

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

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

In the next patches the kernel stack size will not be a compile
time constant and would be less than or equal THREAD_SIZE based
on a command line arg. Therefore, use vm_area->nr_pages to
determine the actual number of pages allocated in stack.

Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>
[Rebased, also skipped intermediary helper variable nr_pages]
Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
Signed-off-by: David Stevens <stevensd at google.com>
[Update commit message]
Signed-off-by: Mostafa Saleh <smostafa at google.com>
---
 kernel/fork.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 5ef413368912..2065693e762c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -315,9 +315,7 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm_area)
 	int ret;
 	int nr_charged = 0;
 
-	BUG_ON(vm_area->nr_pages != THREAD_SIZE / PAGE_SIZE);
-
-	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
+	for (i = 0; i < vm_area->nr_pages; i++) {
 		ret = memcg_kmem_charge_page(vm_area->pages[i], GFP_KERNEL, 0);
 		if (ret)
 			goto err;
@@ -488,7 +486,7 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 		struct vm_struct *vm_area = task_stack_vm_area(tsk);
 		int i;
 
-		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+		for (i = 0; i < vm_area->nr_pages; i++)
 			mod_lruvec_page_state(vm_area->pages[i], NR_KERNEL_STACK_KB,
 					      account * (PAGE_SIZE / 1024));
 	} else {
@@ -509,7 +507,7 @@ void exit_task_stack_account(struct task_struct *tsk)
 		int i;
 
 		vm_area = task_stack_vm_area(tsk);
-		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+		for (i = 0; i < vm_area->nr_pages; i++)
 			memcg_kmem_uncharge_page(vm_area->pages[i], 0);
 	}
 }
-- 
2.56.0.rc1.315.gc6ed9934b7-goog