FAILED: Patch "mm/vmalloc: prevent RCU stalls in kasan_release_vmalloc_node" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 5747435e0fd474c24530ef1a6822f47e7d264b27 Mon Sep 17 00:00:00 2001
From: Deepanshu Kartikey <[email protected]>
Date: Mon, 12 Jan 2026 16:06:12 +0530
Subject: [PATCH] mm/vmalloc: prevent RCU stalls in kasan_release_vmalloc_node

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.

---
 mm/vmalloc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

index 32d6ee92d4ff8..ca4c653286870 100644
@@ -2273,11 +2273,14 @@ 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;

@@ -2287,6 +2290,11 @@ 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);

2.51.0