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

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Date:	Fri, 11 May 2012 00:00:42 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	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 10:47:31AM -0400, Nitin Gupta wrote:
> On 5/10/12 10:02 AM, Konrad Rzeszutek Wilk wrote:
> >On Thu, May 10, 2012 at 11:03:19AM +0900, Minchan Kim wrote:
> >>On 05/10/2012 05:19 AM, Greg Kroah-Hartman wrote:
> >>
> >>>On Fri, May 04, 2012 at 11:24:54AM +0900, Minchan Kim wrote:
> >>>>On 05/04/2012 12:23 AM, Seth Jennings wrote:
> >>>>
> >>>>>On 05/03/2012 08:32 AM, Nitin Gupta wrote:
> >>>>>
> >>>>>>On 5/3/12 2:40 AM, Minchan Kim wrote:
> >>>>>>>We should use zs_handle instead of void * to avoid any
> >>>>>>>confusion. Without this, users may just treat zs_malloc return value as
> >>>>>>>a pointer and try to deference it.
> >>>>>>>
> >>>>>>>Cc: Dan Magenheimer<dan.magenheimer@...cle.com>
> >>>>>>>Cc: Konrad Rzeszutek Wilk<konrad.wilk@...cle.com>
> >>>>>>>Signed-off-by: Minchan Kim<minchan@...nel.org>
> >>>>>>>---
> >>>>>>>   drivers/staging/zcache/zcache-main.c     |    8 ++++----
> >>>>>>>   drivers/staging/zram/zram_drv.c          |    8 ++++----
> >>>>>>>   drivers/staging/zram/zram_drv.h          |    2 +-
> >>>>>>>   drivers/staging/zsmalloc/zsmalloc-main.c |   28
> >>>>>>>++++++++++++++--------------
> >>>>>>>   drivers/staging/zsmalloc/zsmalloc.h      |   15 +++++++++++----
> >>>>>>>   5 files changed, 34 insertions(+), 27 deletions(-)
> >>>>>>
> >>>>>>This was a long pending change. Thanks!
> >>>>>
> >>>>>
> >>>>>The reason I hadn't done it before is that it introduces a checkpatch
> >>>>>warning:
> >>>>>
> >>>>>WARNING: do not add new typedefs
> >>>>>#303: FILE: drivers/staging/zsmalloc/zsmalloc.h:19:
> >>>>>+typedef void * zs_handle;
> >>>>>
> >>>>
> >>>>
> >>>>Yes. I did it but I think we are (a) of chapter 5: Typedefs in Documentation/CodingStyle.
> >>>>
> >>>>  (a) totally opaque objects (where the typedef is actively used to _hide_
> >>>>      what the object is).
> >>>>
> >>>>No?
> >>>
> >>>No.
> >>>
> >>>Don't add new typedefs to the kernel.  Just use a structure if you need
> >>>to.
> >>
> >>
> >>I tried it but failed because there were already tightly coupling between [zcache|zram]
> >>and zsmalloc.
> >>They already knows handle's internal well so they used it as pointer, even zcache keeps
> >>handle's value as some key in tmem_put and tmem_get
> >>AFAIK, ramster also will use zsmalloc sooner or later and add more coupling codes. Sigh.
> >>Please fix it as soon as possible.
> >>
> >>Dan, Seth
> >>Any ideas?
> >
> >struct zs {

I like struct zs_handle.

> >	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

That was my point and I think it's not minor problem.

> zs'. This will unnecessarily waste sizeof(void *) for every object
> stored.
> 
> We could 'memcpy' struct zs to a void * and then store that directly

I don't like it because it still coupled with zsmalloc which means
zcache already know zs's internal so we should avoid it.

> in the radix node but not sure if that would be less ugly than just
> returning the handle as a void * as is done currently.
> 
> Thanks,
> Nitin
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