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

    thanks,

    greg k-h


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