Date: Thu, 10 May 2012 09:44:18 -0700
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Nitin Gupta <ngupta@vflare.org>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Minchan Kim <minchan@kernel.org>,
        Dan Magenheimer <dan.magenheimer@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 3/4] zsmalloc use zs_handle instead of void *
Message-ID: <20120510164418.GC13964@kroah.com>
References: <4FA28907.9020300@vflare.org>
 <4FA2A2F0.3030509@linux.vnet.ibm.com>
 <4FA33DF6.8060107@kernel.org>
 <20120509201918.GA7288@kroah.com>
 <4FAB21E7.7020703@kernel.org>
 <20120510140215.GC26152@phenom.dumpdata.com>
 <4FABD503.4030808@vflare.org>
 <4FABDA9F.1000105@linux.vnet.ibm.com>
 <20120510151941.GA18302@kroah.com>
 <4FABECF5.8040602@vflare.org>
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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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