[PATCH v2 2/2] kthread: Report cpumask allocation failure without warning

[Quchaosheng]

kthread_affine_node() warns when zalloc_cpumask_var() fails:

  if (!zalloc_cpumask_var(&affinity, GFP_KERNEL)) {
          WARN_ON_ONCE(1);
          return;
  }

The allocation uses GFP_KERNEL, so it can fail under memory pressure or
fault injection.  A failed allocation is a recoverable condition and not a
kernel bug, so the warning is noise.  syzbot reports it for a WireGuard
NAPI thread:

  WARNING: kernel/kthread.c:359 at kthread_affine_node+0x200/0x2e8
  CPU: 0 PID: 5207 Comm: napi/wg2-0
  Call Trace:
   alloc_cpumask_var_node+0xfc/0x138
   zalloc_cpumask_var
   kthread_affine_node+0x148/0x2e8
   kthread+0x29c/0x3d4
   ret_from_fork+0x10/0x20

The failure is not silent though: the early return also skips
list_add_tail() of kthread::affinity_node, so the thread never joins
kthread_affinity_list and kthreads_online_cpu() will not fix up its
affinity on a later CPU hotplug.  Keep the failure visible with
pr_warn_once() instead of dropping the message.

Reported-by: syzbot+37ca7ae3e98cb65c3209@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=37ca7ae3e98cb65c3209
Signed-off-by: Quchaosheng <quchaosheng000406@xxxxxxx>
---
v2:
 - Keep the failure visible instead of dropping the message silently
   (Valentine Schneider, Waiman Long).  A WARN is not appropriate for a
   recoverable GFP_KERNEL allocation failure, but the early return also
   keeps the thread out of kthread_affinity_list, so report it with
   pr_warn_once().

 kernel/kthread.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index a3f95c904..b59fa7c7e 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -356,7 +356,14 @@ static void kthread_affine_node(void)
 		return;
 
 	if (!zalloc_cpumask_var(&affinity, GFP_KERNEL)) {
-		WARN_ON_ONCE(1);
+		/*
+		 * The thread stays out of kthread_affinity_list, so a later
+		 * CPU hotplug will not fix up its affinity. Report it, but do
+		 * not warn: the allocation can fail under memory pressure or
+		 * fault injection, and that is not a kernel bug.
+		 */
+		pr_warn_once("kthread: %s: no cpumask, node affinity not set\n",
+			     current->comm);
 		return;
 	}