From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757462Ab2EJRYk (ORCPT ); Thu, 10 May 2012 13:24:40 -0400 Received: from mail-qa0-f49.google.com ([209.85.216.49]:57140 "EHLO mail-qa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755988Ab2EJRYi (ORCPT ); Thu, 10 May 2012 13:24:38 -0400 Message-ID: <4FABF9D4.8080303@vflare.org> Date: Thu, 10 May 2012 13:24:36 -0400 From: Nitin Gupta User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:12.0) Gecko/20120428 Thunderbird/12.0.1 MIME-Version: 1.0 To: Greg Kroah-Hartman CC: Seth Jennings , Konrad Rzeszutek Wilk , Minchan Kim , Dan Magenheimer , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 3/4] zsmalloc use zs_handle instead of void * 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> <20120510164418.GC13964@kroah.com> In-Reply-To: <20120510164418.GC13964@kroah.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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