[PATCH 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
[Rebased, also skipped intermediary helper variable nr_pages]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 kernel/fork.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 2350ba55db2a5f5675f810d7214252dd8a1fae98..7710b7ded3000c77e8d0871bb6bb5916d90aa4ec 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -271,7 +271,7 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 {
 	struct vm_struct *vm_area;
 	void *stack;
-	int i;
+	int i, j;
 
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
 		vm_area = this_cpu_xchg(cached_stacks[i], NULL);
@@ -289,7 +289,8 @@ 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 < vm_area->nr_pages; j++)
+			clear_page(page_address(vm_area->pages[j]));
 
 		tsk->stack_vm_area = vm_area;
 		tsk->stack = stack;

-- 
2.49.0