[PATCH v2] mm: add VM_STATIC flag to vmalloc and prevent from removing the areas

[Hoeun Ryu]

 vm_area_add_early/vm_area_register_early() are used to reserve vmalloc area
during boot process and those virtually mapped areas are never unmapped.
So `OR` VM_STATIC flag to the areas in vmalloc_init() when importing
existing vmlist entries and prevent those areas from being removed from the
rbtree by accident. This flags can be also used by other vmalloc APIs to
specify that the area will never go away.
 This makes remove_vm_area() more robust against other kind of errors (eg.
programming errors).

Signed-off-by: Hoeun Ryu <hoeun.ryu@xxxxxxxxx>
---
v2:
 - update changelog
 - add description to VM_STATIC
 - check VM_STATIC first before VM_VM_AREA in remove_vm_area()

 include/linux/vmalloc.h | 7 +++++++
 mm/vmalloc.c            | 9 ++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 46991ad..a42c210 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -19,6 +19,13 @@ struct notifier_block;		/* in notifier.h */
 #define VM_UNINITIALIZED	0x00000020	/* vm_struct is not fully initialized */
 #define VM_NO_GUARD		0x00000040      /* don't add guard page */
 #define VM_KASAN		0x00000080      /* has allocated kasan shadow memory */
+/*
+ * static area, vmap_area will never go away.
+ * This flag is OR-ed automatically in vmalloc_init() if the area is inserted
+ * by vm_area_add_early()/vm_area_register_early() during early boot process
+ * or you can give this flag manually using other vmalloc APIs.
+ */
+#define VM_STATIC		0x00000200
 /* bits [20..32] reserved for arch specific ioremap internals */
 
 /*
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 8ef8ea1..6bc6c39 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1262,7 +1262,7 @@ void __init vmalloc_init(void)
 	/* Import existing vmlist entries. */
 	for (tmp = vmlist; tmp; tmp = tmp->next) {
 		va = kzalloc(sizeof(struct vmap_area), GFP_NOWAIT);
-		va->flags = VM_VM_AREA;
+		va->flags = VM_VM_AREA | VM_STATIC;
 		va->va_start = (unsigned long)tmp->addr;
 		va->va_end = va->va_start + tmp->size;
 		va->vm = tmp;
@@ -1480,7 +1480,7 @@ struct vm_struct *remove_vm_area(const void *addr)
 	might_sleep();
 
 	va = find_vmap_area((unsigned long)addr);
-	if (va && va->flags & VM_VM_AREA) {
+	if (va && likely(!(va->flags & VM_STATIC)) && va->flags & VM_VM_AREA) {
 		struct vm_struct *vm = va->vm;
 
 		spin_lock(&vmap_area_lock);
@@ -1510,7 +1510,7 @@ static void __vunmap(const void *addr, int deallocate_pages)
 
 	area = remove_vm_area(addr);
 	if (unlikely(!area)) {
-		WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
+		WARN(1, KERN_ERR "Trying to vfree() nonexistent or static vm area (%p)\n",
 				addr);
 		return;
 	}
@@ -2708,6 +2708,9 @@ static int s_show(struct seq_file *m, void *p)
 	if (v->phys_addr)
 		seq_printf(m, " phys=%pa", &v->phys_addr);
 
+	if (v->flags & VM_STATIC)
+		seq_puts(m, " static");
+
 	if (v->flags & VM_IOREMAP)
 		seq_puts(m, " ioremap");
 
-- 
2.7.4