[PATCH 1/1] fork,memcg: alloc_thread_stack_node needs to set tsk->stack

[Andrea Arcangeli <aarcange@redhat.com>]

Commit 5eed6f1dff87bfb5e545935def3843edf42800f2 corrected two
instances, but there was a third instance of this bug.

Without setting tsk->stack, if memcg_charge_kernel_stack fails, it'll
execute free_thread_stack() on a dangling pointer.

Enterprise kernels are compiled with VMAP_STACK=y so this isn't
critical, but custom VMAP_STACK=n builds should have some performance
advantage, with the drawback of risking to fail fork because
compaction didn't succeed. So as long as VMAP_STACK=n is a supported
option it's worth fixing it upstream.

Fixes: 9b6f7e163cd0 ("mm: rework memcg kernel stack accounting")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 kernel/fork.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index d6c324b1b29e..9ee28dfe7c21 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -248,7 +248,11 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 	struct page *page = alloc_pages_node(node, THREADINFO_GFP,
 					     THREAD_SIZE_ORDER);
 
-	return page ? page_address(page) : NULL;
+	if (likely(page)) {
+		tsk->stack = page_address(page);
+		return tsk->stack;
+	}
+	return NULL;
 #endif
 }