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

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Date:   Thu, 13 Apr 2017 09:47:03 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Hoeun Ryu <hoeun.ryu@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Ingo Molnar <mingo@...nel.org>, zijun_hu <zijun_hu@....com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add VM_STATIC flag to vmalloc and prevent from
 removing the areas

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@...il.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.