linux-kernel - Re: No more bits in vm_area_struct's vm

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Message-ID: <a0ea21a7cfe313202e2b51510aa5435a.squirrel@webmail-b.css.fujitsu.com>
Date:	Tue, 29 Sep 2009 01:14:03 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Hugh Dickins" <hugh.dickins@...cali.co.uk>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Wu Fengguang" <fengguang.wu@...el.com>,
	"Nigel Cunningham" <ncunningham@...a.org.au>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: No more bits in vm_area_struct's vm_flags.

Hugh Dickins wrote:
> On Mon, 28 Sep 2009, KAMEZAWA Hiroyuki wrote:
>>
>> What I dislike is making vm_flags to be long long ;)
>
> Why?
I'm sorry if my "dislike" sounds too strong.

Every time I see long long in the kernel, my concern is
"do I need spinlock to access this for 32bit arch ? is it safe ?".
(And it makes binary=>disassemble=>C (by eyes) a bit difficult)
Then, I don't like long long personally.

Another reason is some other calls like test_bit() cannot be used against
long long. (even if it's not used _now_)

Maybe vm->vm_flags will not require extra locks because
it can be protected by bigger lock as mmap_sem. Then, please make it
to be long long if its's recommended.

keeping vm_flags to be 32bit may makes vma_merge() ugly.
If so, long long is  a choice.

Thanks,
-Kame

> Hugh
>

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