Re: [PATCH v2] mm/vmalloc: prevent RCU stalls in kasan_release_vmalloc_node

[Hillf Danton <hdanton@sina.com>]

#syz test

When CONFIG_PAGE_OWNER is enabled, freeing KASAN shadow pages during
vmalloc cleanup triggers expensive stack unwinding that acquires RCU
read locks. Processing a large purge_list without rescheduling can
cause the task to hold CPU for extended periods (10+ seconds), leading
to RCU stalls and potential OOM conditions.

The issue manifests in purge_vmap_node() -> kasan_release_vmalloc_node()
where iterating through hundreds or thousands of vmap_area entries and
freeing their associated shadow pages causes:

  rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
  rcu: Tasks blocked on level-0 rcu_node (CPUs 0-1): P6229/1:b..l
  ...
  task:kworker/0:17 state:R running task stack:28840 pid:6229
  ...
  kasan_release_vmalloc_node+0x1ba/0xad0 mm/vmalloc.c:2299
  purge_vmap_node+0x1ba/0xad0 mm/vmalloc.c:2299

Each call to kasan_release_vmalloc() can free many pages, and with
page_owner tracking, each free triggers save_stack() which performs
stack unwinding under RCU read lock. Without yielding, this creates
an unbounded RCU critical section.

Add periodic cond_resched() calls within the loop to allow:
- RCU grace periods to complete
- Other tasks to run
- Scheduler to preempt when needed

The fix uses need_resched() for immediate response under load, with
a batch count of 32 as a guaranteed upper bound to prevent worst-case
stalls even under light load.

Reported-by: syzbot+d8d4c31d40f868eaea30@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d8d4c31d40f868eaea30
Link: https://lore.kernel.org/all/20260112084723.622910-1-kartikey406@gmail.com/T/ [v1]
Suggested-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
v2: Use a macro for batch size (suggested by Uladzislau Rezki)
---
 mm/vmalloc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 41dd01e8430c..51e58701565d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2268,11 +2268,14 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
 	reclaim_list_global(&decay_list);
 }
 
+#define KASAN_RELEASE_BATCH_SIZE 32
+
 static void
 kasan_release_vmalloc_node(struct vmap_node *vn)
 {
 	struct vmap_area *va;
 	unsigned long start, end;
+	unsigned int batch_count = 0;
 
 	start = list_first_entry(&vn->purge_list, struct vmap_area, list)->va_start;
 	end = list_last_entry(&vn->purge_list, struct vmap_area, list)->va_end;
@@ -2282,6 +2285,11 @@ kasan_release_vmalloc_node(struct vmap_node *vn)
 			kasan_release_vmalloc(va->va_start, va->va_end,
 				va->va_start, va->va_end,
 				KASAN_VMALLOC_PAGE_RANGE);
+
+		if (need_resched() || (++batch_count >= KASAN_RELEASE_BATCH_SIZE)) {
+			cond_resched();
+			batch_count = 0;
+		}
 	}
 
 	kasan_release_vmalloc(start, end, start, end, KASAN_VMALLOC_TLB_FLUSH);
-- 
2.43.0