linux-kernel - Re: [PATCH 3/4] zsmalloc use zs

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Date:	Thu, 10 May 2012 09:44:18 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [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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