From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760937Ab2EJRjv (ORCPT ); Thu, 10 May 2012 13:39:51 -0400 Received: from rcsinet15.oracle.com ([148.87.113.117]:37222 "EHLO rcsinet15.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760717Ab2EJRjq (ORCPT ); Thu, 10 May 2012 13:39:46 -0400 Date: Thu, 10 May 2012 13:33:22 -0400 From: Konrad Rzeszutek Wilk To: Nitin Gupta Cc: Greg Kroah-Hartman , Seth Jennings , 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 * Message-ID: <20120510173322.GA30481@phenom.dumpdata.com> References: <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> <4FABF9D4.8080303@vflare.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4FABF9D4.8080303@vflare.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-Source-IP: ucsinet21.oracle.com [156.151.31.93] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 10, 2012 at 01:24:36PM -0400, Nitin Gupta wrote: > 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. It is the same size: sizeof(struct zs) == sizeof(void *). When you return the 'struct zs' it will be as if you are returning a void * 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