linux-kernel - Re: [RFC PATCH] mm/zsmalloc: remove unnecessary check

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Date:	Fri, 21 Nov 2014 19:32:32 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Ganesh Mahendran <opensource.ganesh@...il.com>
Cc:	Nitin Gupta <ngupta@...are.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm/zsmalloc: remove unnecessary check

On Fri, Nov 21, 2014 at 06:48:49AM +0000, Minchan Kim wrote:
> On Fri, Nov 21, 2014 at 01:33:26PM +0800, Ganesh Mahendran wrote:
> > Hello
> > 
> > 2014-11-21 11:54 GMT+08:00 Minchan Kim <minchan@...nel.org>:
> > > On Thu, Nov 20, 2014 at 09:21:56PM +0800, Mahendran Ganesh wrote:
> > >> ZS_SIZE_CLASSES is calc by:
> > >>   ((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / ZS_SIZE_CLASS_DELTA + 1)
> > >>
> > >> So when i is in [0, ZS_SIZE_CLASSES - 1), the size:
> > >>   size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA
> > >> will not be greater than ZS_MAX_ALLOC_SIZE
> > >>
> > >> This patch removes the unnecessary check.
> > >
> > > It depends on ZS_MIN_ALLOC_SIZE.
> > > For example, we would change min to 8 but MAX is still 4096.
> > > ZS_SIZE_CLASSES is (4096 - 8) / 16 + 1 = 256 so 8 + 255 * 16 = 4088,
> > > which exceeds the max.
> > Here, 4088 is less than MAX(4096).
> > 
> > ZS_SIZE_CLASSES = (MAX - MIN) / Delta + 1
> > So, I think the value of
> >     MIN + (ZS_SIZE_CLASSES - 1) * Delta =
> >     MIN + ((MAX - MIN) / Delta) * Delta =
> >     MAX
> > will not exceed the MAX
> 
> You're right. It was complext math for me.
> I should go back to elementary school.
> 
> Thanks!
> 
> Acked-by: Minchan Kim <minchan@...nel.org>

I catch a nasty cold but above my poor math makes me think more.
ZS_SIZE_CLASSES is broken. In above my example, current code cannot
allocate 4096 size class so we should correct ZS_SIZE_CLASSES
at first.

zs_size_classes = zs_max - zs_min / delta + 1;
if ((zs_max - zs_min) % delta)
	zs_size_classes += 1;

Then, we need to code piece you removed.
As well, we need to fix below.

- area->vm_buf = (char *)__get_free_page(GFP_KERNEL);
+ area->vm_buf = kmalloc(ZS_MAX_ALLOC_SIZE);

Hope I am sane in this time :(
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