[RFC][PATCH 02/24] kernel/fork: Use sizeof() union instead of TASK_SIZE

[Dave Hansen]

thread_union is sized with TASK_SIZE. Instead of needing the definition
and this code to match, just use the thread_union size. This way, if the
thread union size changes, this code will still be correct.

Foreshadowing: shadow stack support will change the size of thread_union.

---

 b/kernel/fork.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff -puN kernel/fork.c~alloc_thread_stack_node-2 kernel/fork.c
--- a/kernel/fork.c~alloc_thread_stack_node-2	2026-10-21 12:36:33.082871786 -0700
+++ b/kernel/fork.c	2026-10-21 12:36:33.085871792 -0700
@@ -315,7 +315,7 @@ static int memcg_charge_kernel_stack(str
 	int ret;
 	int nr_charged = 0;
 
-	BUG_ON(vm_area->nr_pages != THREAD_SIZE / PAGE_SIZE);
+	BUG_ON(vm_area->nr_pages != sizeof(union thread_union) / PAGE_SIZE);
 
 	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
 		ret = memcg_kmem_charge_page(vm_area->pages[i], GFP_KERNEL, 0);
@@ -332,7 +332,7 @@ err:
 
 static void *__vmalloc_thread_stack_node(int node)
 {
-	return __vmalloc_node(THREAD_SIZE, THREAD_ALIGN,
+	return __vmalloc_node(sizeof(union thread_union), THREAD_ALIGN,
 			      GFP_VMAP_STACK,
 			      node, __builtin_return_address(0));
 }
_