Re: [PATCH 3/4] zsmalloc use zs_handle instead of void *

[Nitin Gupta <ngupta@xxxxxxxxxx>]

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@xxxxxxxxxx>
> > > > > > > Cc: Konrad Rzeszutek Wilk<konrad.wilk@xxxxxxxxxx>
> > > > > > > Signed-off-by: Minchan Kim<minchan@xxxxxxxxxx>
> > > > > > > ---
> > > > > > >    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 {
> 	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.

We could 'memcpy' struct zs to a void * and then store that directly 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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