linux-kernel - Re: [RFC] ps command race fix

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Message-Id: <20060725115004.a6c668ca.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 25 Jul 2006 11:50:04 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	pj@....com, linux-kernel@...r.kernel.org, ebiederm@...ssion.com
Subject: Re: [RFC] ps command race fix

On Mon, 24 Jul 2006 19:33:18 -0700
Andrew Morton <akpm@...l.org> wrote:

> On Tue, 25 Jul 2006 11:08:35 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > > Then the seek and read and such semantics are nice and stable and
> > > simple.
> > > 
> > yes...
> > I think snapshot at open() is okay.
> 
> We cannot do a single kmalloc() like cpuset does.
> 
> The kernel presently kind-of guarantees that a 32k kmalloc() will work,
> although the VM might have to do very large amounts of work to achieve it.
> 
> But 32k is only 8192 processes, so a snapshot will need multiple
> allocations and a list and trouble dropping and retaking tasklist_lock to
> allocate memory and keeping things stable while doing that.  I suspect
> it'll end up ugly.
> 

Hm, how about using bitmap instead of table ? (we'll have many holes but...)
Implementing
- sytem-wide bitmap of used tgid which is updated only when /proc is opened
seems not to be much problem. 32k kmalloc can store 256k pids.

BTW, how large pids and how many proccess in a (heavy and big) system ?

-Kame

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