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

[Nitin Gupta]

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.

Thanks,
Nitin

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