Re: [PATCH v2 2/3] mm/vmalloc: fix vmalloc_dump_obj cross-zone VA lookup

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Sep 21, 2026 at 09:17:22PM +0800, Ye Liu wrote:
> From: Ye Liu <liuye@kylinos.cn>
> 
> vmalloc_dump_obj() searches only one vmap node (addr_to_node(addr)),
> but a vmalloc allocation may span multiple vmap zones.  The VA is
> stored in only one node's rb-tree (addr_to_node(va_start)), so an
> object pointer in a different zone than va_start maps to a different
> node and the search misses.  This affects any allocation larger than
> vmap_zone_size (64 KiB) on multi-CPU systems.
> 
> Iterate all vmap nodes using for_each_vmap_node, like find_vmap_area()
> does, but with spin_trylock instead of spin_lock as this function can
> be called from atomic dump contexts (OOM, KASAN, RCU).
> 
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
> ---
>  mm/vmalloc.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index df42d8a6f058..30c610f678dc 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -5278,17 +5278,29 @@ bool vmalloc_dump_obj(void *object)
>  	unsigned long nr_pages;
>  
>  	addr = PAGE_ALIGN_DOWN((unsigned long) object);
> -	vn = addr_to_node(addr);
>  
> -	if (!spin_trylock(&vn->busy.lock))
> -		return false;
> +	/*
> +	 * A vmalloc allocation may span multiple vmap zones, so the
> +	 * node whose rb-tree holds the VA may differ from the node
> +	 * the address maps to.  Search all nodes.  Use trylock as
> +	 * this function can be called from atomic dump contexts.
> +	 */
> +	va = NULL;
> +	for_each_vmap_node(vn) {
> +		if (!spin_trylock(&vn->busy.lock))
> +			continue;
> +
> +		va = __find_vmap_area(addr, &vn->busy.root);
> +		if (va && va->vm)
> +			break;
>  
> -	va = __find_vmap_area(addr, &vn->busy.root);
> -	if (!va || !va->vm) {
>  		spin_unlock(&vn->busy.lock);
> -		return false;
> +		va = NULL;
>  	}
>  
> +	if (!va)
> +		return false;
> +
>  	vm = va->vm;
>  	addr = (unsigned long) vm->addr;
>  	caller = vm->caller;
> 
> -- 
> 2.25.1
> 

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 89c327a6ce7d..3719dc02dcaf 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2511,7 +2511,22 @@ static void free_unmap_vmap_area(struct vmap_area *va)
 	free_vmap_area_noflush(va);
 }
 
-struct vmap_area *find_vmap_area(unsigned long addr)
+static inline int next_vmap_node_id(int i)
+{
+	return (i + nr_vmap_nodes - 1) % nr_vmap_nodes;
+}
+
+enum vmap_lock_mode {
+	VMAP_LOCK,
+	VMAP_TRYLOCK,
+};
+
+/*
+ * Add a comment here.
+ */
+static struct vmap_area *
+find_vmap_area_lock(unsigned long addr, struct vmap_node **locked_vn,
+		enum vmap_lock_mode mode)
 {
 	struct vmap_node *vn;
 	struct vmap_area *va;
@@ -2534,16 +2549,40 @@ struct vmap_area *find_vmap_area(unsigned long addr)
 	 * addr is within 2 or 0 nodes we should do extra work.
 	 */
 	i = j = addr_to_node_id(addr);
+
 	do {
 		vn = &vmap_nodes[i];
 
-		spin_lock(&vn->busy.lock);
+		if (mode == VMAP_LOCK) {
+			spin_lock(&vn->busy.lock);
+		} else {
+			if (!spin_trylock(&vn->busy.lock))
+				continue;
+		}
+
 		va = __find_vmap_area(addr, &vn->busy.root);
+		if (va) {
+			*locked_vn = vn;
+			return va;
+		}
+
 		spin_unlock(&vn->busy.lock);
+	} while ((i = next_vmap_node_id(i)) != j);
 
-		if (va)
-			return va;
-	} while ((i = (i + nr_vmap_nodes - 1) % nr_vmap_nodes) != j);
+	*locked_vn = NULL;
+	return NULL;
+}
+
+struct vmap_area *find_vmap_area(unsigned long addr)
+{
+	struct vmap_node *vn;
+	struct vmap_area *va;
+
+	va = find_vmap_area_lock(addr, &vn, VMAP_LOCK);
+	if (va) {
+		spin_unlock(&vn->busy.lock);
+		return va;
+	}
 
 	return NULL;
 }


and we use the helper in the vmalloc_dump_obj()?

--
Uladzislau Rezki