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

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Message-ID: <20141121064849.GA17181@gmail.com>
Date:	Fri, 21 Nov 2014 06:48:49 +0000
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 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>


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