From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932872Ab2EJXYd (ORCPT ); Thu, 10 May 2012 19:24:33 -0400 Received: from LGEMRELSE1Q.lge.com ([156.147.1.111]:44575 "EHLO LGEMRELSE1Q.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752118Ab2EJXYb (ORCPT ); Thu, 10 May 2012 19:24:31 -0400 X-AuditID: 9c93016f-b7cecae000000e00-90-4fac4e2d0130 Message-ID: <4FAC4E3B.3030909@kernel.org> Date: Fri, 11 May 2012 08:24:43 +0900 From: Minchan Kim User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120410 Thunderbird/11.0.1 MIME-Version: 1.0 Newsgroups: gmane.linux.kernel.mm,gmane.linux.kernel To: Konrad Rzeszutek Wilk CC: Nitin Gupta , Greg Kroah-Hartman , Seth Jennings , 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: <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> <20120510173322.GA30481@phenom.dumpdata.com> In-Reply-To: <20120510173322.GA30481@phenom.dumpdata.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Brightmail-Tracker: AAAAAA== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/11/2012 02:33 AM, Konrad Rzeszutek Wilk wrote: > 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. > Please look. struct zs_handle { void *handle }; 1) static struct zv_hdr *zv_create(..) { struct zs_handle handle; .. handle = zs_malloc(pool, size); .. return handle; } handle is on stack so it can't be used by index for slot of radix tree. 2) static struct zv_hdr *zv_create(..) { struct zs_handle handle; .. handle = zs_malloc(pool, size); .. return handle.handle; } Okay. Now it works but zcache coupled with zsmalloc tightly. User of zsmalloc should never know internal of zs_handle. 3) - zsmalloc.h void *zs_handle_to_ptr(struct zs_handle handle) { return handle.hanle; } static struct zv_hdr *zv_create(..) { struct zs_handle handle; .. handle = zs_malloc(pool, size); .. return zs_handle_to_ptr(handle); } Why should zsmalloc support such interface? It's a zcache problem so it's desriable to solve it in zcache internal. And in future, if we can add/remove zs_handle's fields, we can't make sure such API. >> 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 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ > Don't email: email@kvack.org > -- Kind regards, Minchan Kim