Re: [PATCH] zsmalloc: fix migrate_zspage-zs_free race condition

[Minchan Kim]

Hi Junil,

On Fri, Jan 15, 2016 at 09:36:24AM +0900, Junil Lee wrote:
> To prevent unlock at the not correct situation, tagging the new obj to
> assure lock in migrate_zspage() before right unlock path.
> 
> Two functions are in race condition by tag which set 1 on last bit of
> obj, however unlock succrently when update new obj to handle before call
> unpin_tag() which is right unlock path.
> 
> summarize this problem by call flow as below:
> 
> 		CPU0								CPU1
> migrate_zspage
> find_alloced_obj()
> 	trypin_tag() -- obj |= HANDLE_PIN_BIT
> obj_malloc() -- new obj is not set			zs_free
> record_obj() -- unlock and break sync		pin_tag() -- get lock
> unpin_tag()

It's really good catch!
I think it should be stable material. For that, we should know this
patch fixes what kinds of problem.

What do you see problem? I mean please write down the oops you saw and
verify that the patch fixes your problem. :)

Minor nit below

> 
> Signed-off-by: Junil Lee <junil0814.lee@xxxxxxx>
> ---
>  mm/zsmalloc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index e7414ce..bb459ef 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1635,6 +1635,7 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
>  		free_obj = obj_malloc(d_page, class, handle);
>  		zs_object_copy(free_obj, used_obj, class);
>  		index++;
> +		free_obj |= BIT(HANDLE_PIN_BIT);
>  		record_obj(handle, free_obj);

I think record_obj should store free_obj to *handle with masking off least bit.
IOW, how about this?

record_obj(handle, obj)
{
        *(unsigned long)handle = obj & ~(1<<HANDLE_PIN_BIT);
}

Thanks a lot!