linux-kernel - Re: [PATCH] memcg: use do_div to divide s64 in 32 bit machine.

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Message-ID: <AANLkTin9m65JVKRuStZ1-qhU5_1AY-GcbBRC0TodsfYC@mail.gmail.com>
Date:	Sat, 6 Nov 2010 10:19:35 -0700
From:	Greg Thelen <gthelen@...gle.com>
To:	hannes@...xchg.org
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <hidave.darkstar@...il.com>,
	Andrea Righi <arighi@...eler.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] memcg: use do_div to divide s64 in 32 bit machine.

On Fri, Nov 5, 2010 at 6:03 PM,  <hannes@...xchg.org> wrote:
> On Sat, Nov 06, 2010 at 01:08:53AM +0900, Minchan Kim wrote:
>> Use do_div to divide s64 value. Otherwise, build would be failed
>> like Dave Young reported.
>
> I thought about that too, but then I asked myself why you would want
> to represent a number of pages as signed 64bit type, even on 32 bit?

I think the reason that 64 byte type is used for page count in
memcontrol.c is because the low level res_counter primitives operate
on 64 bit counters, even on 32 bit machines.

> Isn't the much better fix to get the types right instead?
>

I agree that consistent types between mem_cgroup_dirty_info() and
global_dirty_info() is important.  There seems to be a lot of usage of
s64 for page counts in memcontrol.c, which I think is due to the
res_counter types.  I think these s64 be switched to unsigned long
rather to be consistent with the rest of mm code.  It looks like this
will be a clean patch, except for the lowest level where
res_counter_read_u64() is used, where some casting may be needed.

I'll post a patch for that change.
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