Re: [PATCH] mm: Fix incorrect pfn passed to untrack_pfn in remap_pfn_range

[Andrew Morton]

On Fri, 22 Apr 2016 18:31:28 +0800 Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:

> We used generic hooks in remap_pfn_range to help archs to
> track pfnmap regions. The code is something like:
> 
> int remap_pfn_range()
> {
> 	...
> 	track_pfn_remap(vma, &prot, pfn, addr, PAGE_ALIGN(size));
> 	...
> 	pfn -= addr >> PAGE_SHIFT;
> 	...
> 	untrack_pfn(vma, pfn, PAGE_ALIGN(size));
> 	...
> }
> 
> Here we can easily find the pfn is changed but not recovered
> before untrack_pfn() is called. That's incorrect.

What are the runtime effects of this bug?

> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1755,6 +1755,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>  			break;
>  	} while (pgd++, addr = next, addr != end);
>  
> +	pfn += (end - PAGE_ALIGN(size)) >> PAGE_SHIFT;
>  	if (err)
>  		untrack_pfn(vma, pfn, PAGE_ALIGN(size));

I'm having trouble understanding this.  Wouldn't it be better to simply
save the track_pfn_remap() call's `pfn' arg in a new local variable?