Re: [PATCH v4 4/4] mm/vmalloc: don't assume vmap_area w/o VM_VM_AREA flag is vm_map_ram allocation

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Tue, Sep 03, 2013 at 04:42:21PM +0900, Joonsoo Kim wrote:
>On Tue, Sep 03, 2013 at 03:01:46PM +0800, Wanpeng Li wrote:
>> There is a race window between vmap_area free and show vmap_area information.
>> 
>> 	A                                                B
>> 
>> remove_vm_area
>> spin_lock(&vmap_area_lock);
>> va->flags &= ~VM_VM_AREA;
>> spin_unlock(&vmap_area_lock);
>> 						spin_lock(&vmap_area_lock);
>> 						if (va->flags & (VM_LAZY_FREE | VM_LAZY_FREEZING))
>> 							return 0;
>> 						if (!(va->flags & VM_VM_AREA)) {
>> 							seq_printf(m, "0x%pK-0x%pK %7ld vm_map_ram\n",
>> 								(void *)va->va_start, (void *)va->va_end,
>> 								va->va_end - va->va_start);
>> 							return 0;
>> 						}
>> free_unmap_vmap_area(va);
>> 	flush_cache_vunmap
>> 	free_unmap_vmap_area_noflush
>> 		unmap_vmap_area
>> 		free_vmap_area_noflush
>> 			va->flags |= VM_LAZY_FREE 
>> 
>> The assumption is introduced by commit: d4033afd(mm, vmalloc: iterate vmap_area_list, 
>> instead of vmlist, in vmallocinfo()). This patch fix it by drop the assumption and 
>> keep not dump vm_map_ram allocation information as the logic before that commit.
>> 
>> Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
>> ---
>>  mm/vmalloc.c | 7 -------
>>  1 file changed, 7 deletions(-)
>> 
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 5368b17..62b7932 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -2586,13 +2586,6 @@ static int s_show(struct seq_file *m, void *p)
>>  	if (va->flags & (VM_LAZY_FREE | VM_LAZY_FREEING))
>>  		return 0;
>>  
>> -	if (!(va->flags & VM_VM_AREA)) {
>> -		seq_printf(m, "0x%pK-0x%pK %7ld vm_map_ram\n",
>> -			(void *)va->va_start, (void *)va->va_end,
>> -					va->va_end - va->va_start);
>> -		return 0;
>> -	}
>> -
>>  	v = va->vm;
>>  
>>  	seq_printf(m, "0x%pK-0x%pK %7ld",
>
>Hello, Wanpeng.
>

Hi Joonsoo and Yanfei,

>Did you test this patch?
>
>I guess that, With this patch, if there are some vm_map areas,
>null pointer deference would occurs, since va->vm may be null for it.
>
>And with this patch, if this race really occur, null pointer deference
>would occurs too, since va->vm is set to null in remove_vm_area().
>
>I think that this is not a right fix for this possible race.
>

How about append below to this patch?

if (va->vm)
	v = va->vm;
else 
	return 0;

Regards,
Wanpeng Li 

>Thanks.

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