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

Anshuman Khandual
2017-04-13T04:17:18Z

On 04/12/2017 10:31 AM, Hoeun Ryu wrote:
> 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.

I am wondering whether protection against accidental deletion
of any vmap area should be done in remove_vm_area() function
or the callers should take care of it. But I guess either way
it works.

>
> Signed-off-by: Hoeun Ryu <hoeun.ryu@gmail.com>
> ---
> include/linux/vmalloc.h | 1 +
> mm/vmalloc.c | 9 ++++++---
> 2 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 46991ad..3df53fc 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -19,6 +19,7 @@ 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 */
> +#define VM_STATIC 0x00000200

You might want to add some description in the comment saying
its a sticky VM area which will never go away or something.

> /* bits [20..32] reserved for arch specific ioremap internals */
>
> /*
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 8ef8ea1..fb5049a 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 && va->flags & VM_VM_AREA && likely(!(va->flags & VM_STATIC))) {

You might want to move the VM_STATIC check before the VM_VM_AREA
check so in cases where the former is set we can save one more
conditional check.