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

From: Nitin Gupta
Date: Thu May 10 2012 - 13:24:35 EST

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.

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.


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