[PATCH 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

It is unsafe to dump vmalloc area information when trying to do so from
some contexts. Add a safer trylock version of the same function to do a
best-effort VMA finding and use it from vmalloc_dump_obj().

Reported-by: Zhen Lei <thunder.leizhen@huaweicloud.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: rcu@vger.kernel.org
Cc: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 mm/vmalloc.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 93cf99aba335..dae347e446e6 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1865,6 +1865,18 @@ struct vmap_area *find_vmap_area(unsigned long addr)
 	return va;
 }
 
+static struct vmap_area *find_vmap_area_trylock(unsigned long addr)
+{
+	struct vmap_area *va;
+
+	if (!spin_trylock(&vmap_area_lock))
+		return NULL;
+	va = __find_vmap_area(addr, &vmap_area_root);
+	spin_unlock(&vmap_area_lock);
+
+	return va;
+}
+
 static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
 {
 	struct vmap_area *va;
@@ -2671,6 +2683,27 @@ struct vm_struct *find_vm_area(const void *addr)
 	return va->vm;
 }
 
+/**
+ * try_to_find_vm_area - find a continuous kernel virtual area
+ * @addr:	  base address
+ *
+ * This function is the same as find_vm_area() except that it is
+ * safe to call if vmap_area_lock is already held and returns NULL
+ * if it is. See comments in find_vmap_area() for other details.
+ *
+ * Return: the area descriptor on success or %NULL on failure.
+ */
+static struct vm_struct *try_to_find_vm_area(const void *addr)
+{
+	struct vmap_area *va;
+
+	va = find_vmap_area_trylock((unsigned long)addr);
+	if (!va)
+		return NULL;
+
+	return va->vm;
+}
+
 /**
  * remove_vm_area - find and remove a continuous kernel virtual area
  * @addr:	    base address
@@ -4277,7 +4310,7 @@ bool vmalloc_dump_obj(void *object)
 	struct vm_struct *vm;
 	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
 
-	vm = find_vm_area(objp);
+	vm = try_to_find_vm_area(objp);
 	if (!vm)
 		return false;
 	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
-- 
2.42.0.rc2.253.gd59a3bf2b4-goog