[PATCH v2 5/5] fork: zero vmap stack using clear_page() instead of memset()

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

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

Do not zero the whole span of the stack, but instead only the
pages that are part of the vm_area.

As several architectures have optimized implementations of
clear_page(), this will give the architecture a clear idea
of what is going on and will speed up the clearing operation.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/20240311164638.2015063-7-pasha.tatashin@soleen.com
[linus.walleij@linaro.org: Rebased]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 kernel/fork.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 6ac4674fdf04081fbcbf1eb99167a4c990a58506..d3f000b846c634221c7dafe6e64185a276c5a08b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -271,13 +271,15 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm_area)
 static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 {
 	struct vm_struct *vm_area;
+	int nr_pages;
 	void *stack;
-	int i;
+	int i, j;
 
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
 		vm_area = this_cpu_xchg(cached_stacks[i], NULL);
 		if (!vm_area)
 			continue;
+		nr_pages = vm_area->nr_pages;
 
 		if (memcg_charge_kernel_stack(vm_area)) {
 			vfree(vm_area->addr);
@@ -290,7 +292,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		stack = kasan_reset_tag(vm_area->addr);
 
 		/* Clear stale pointers from reused stack. */
-		memset(stack, 0, THREAD_SIZE);
+		for (j = 0; j < nr_pages; j++)
+			clear_page(page_address(vm_area->pages[j]));
 
 		tsk->stack_vm_area = vm_area;
 		tsk->stack = stack;

-- 
2.49.0