Re: [PATCH 3/4] zsmalloc use zs_handle instead of void *

[Minchan Kim <minchan@kernel.org>]

On 05/11/2012 02:33 AM, Konrad Rzeszutek Wilk wrote:

> On Thu, May 10, 2012 at 01:24:36PM -0400, Nitin Gupta wrote:
>> On 5/10/12 12:44 PM, Greg Kroah-Hartman wrote:
>>> On Thu, May 10, 2012 at 12:29:41PM -0400, Nitin Gupta wrote:
>>>> On 5/10/12 11:19 AM, Greg Kroah-Hartman wrote:
>>>>> On Thu, May 10, 2012 at 10:11:27AM -0500, Seth Jennings wrote:
>>>>>> On 05/10/2012 09:47 AM, Nitin Gupta wrote:
>>>>>>
>>>>>>> On 5/10/12 10:02 AM, Konrad Rzeszutek Wilk wrote:
>>>>>>>> struct zs {
>>>>>>>>     void *ptr;
>>>>>>>> };
>>>>>>>>
>>>>>>>> And pass that structure around?
>>>>>>>>
>>>>>>>
>>>>>>> A minor problem is that we store this handle value in a radix tree node.
>>>>>>> If we wrap it as a struct, then we will not be able to store it directly
>>>>>>> in the node -- the node will have to point to a 'struct zs'. This will
>>>>>>> unnecessarily waste sizeof(void *) for every object stored.
>>>>>>
>>>>>>
>>>>>> I don't think so. You can use the fact that for a struct zs var,&var
>>>>>> and&var->ptr are the same.
>>>>>>
>>>>>> For the structure above:
>>>>>>
>>>>>> void * zs_to_void(struct zs *p) { return p->ptr; }
>>>>>> struct zs * void_to_zs(void *p) { return (struct zs *)p; }
>>>>>
>>>>> Do like what the rest of the kernel does and pass around *ptr and use
>>>>> container_of to get 'struct zs'.  Yes, they resolve to the same pointer
>>>>> right now, but you shouldn't "expect" to to be the same.
>>>>>
>>>>>
>>>>
>>>> I think we can just use unsigned long as zs handle type since all we
>>>> have to do is tell the user that the returned value is not a
>>>> pointer. This will be less pretty than a typedef but still better
>>>> than a single entry struct + container_of stuff.
>>>
>>> But then you are casting the thing all around just as much as you were
>>> with the void *, right?
>>>
>>> Making this a "real" structure ensures type safety and lets the compiler
>>> find the problems you accidentally create at times :)
>>>
>>
>> If we return a 'struct zs' from zs_malloc then I cannot see how we
>> are solving the original problem of storing the handle directly in a
>> radix node. If we pass a struct zs we will require pointing radix
>> node to this struct, wasting sizeof(void *) for every object.   If
>> we pass unsigned long, then this problem is solved and it also makes
>> it clear that the passed value is not a pointer.
> 
> It is the same size: sizeof(struct zs) == sizeof(void *).
> When you return the 'struct zs' it will be as if you are returning 
> a void * pointer.
> 


Please look.

struct zs_handle {
	void *handle
};

1)

static struct zv_hdr *zv_create(..)
{
	struct zs_handle handle;
	..
	handle = zs_malloc(pool, size);
	..
	return handle;
}

handle is on stack so it can't be used by index for slot of radix tree.

2)

static struct zv_hdr *zv_create(..)
{
	struct zs_handle handle;
	..
	handle = zs_malloc(pool, size);
	..
	return handle.handle;
}

Okay. Now it works but zcache coupled with zsmalloc tightly.
User of zsmalloc should never know internal of zs_handle.

3)

- zsmalloc.h
void *zs_handle_to_ptr(struct zs_handle handle)
{
	return handle.hanle;
}

static struct zv_hdr *zv_create(..)
{
	struct zs_handle handle;
	..
	handle = zs_malloc(pool, size);
	..
	return zs_handle_to_ptr(handle);
}

Why should zsmalloc support such interface?
It's a zcache problem so it's desriable to solve it in zcache internal.
And in future, if we can add/remove zs_handle's fields, we can't make
sure such API.


>> Its true that making it a real struct would prevent accidental casts
>> to void * but due to the above problem, I think we have to stick
>> with unsigned long.
>>
>> Thanks,
>> Nitin
> 
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-- 
Kind regards,
Minchan Kim

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