Subject: Re: [Bug #15041] Pagemap endless read loop with LTP
From: Matt Mackall <mpm@selenic.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>,
       =?ISO-8859-1?Q?Am=E9rico?= Wang <xiyou.wangcong@gmail.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Kernel Testers List <kernel-testers@vger.kernel.org>
In-Reply-To: <20100113235010.GF24818@basil.fritz.box>
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	 <201001132257.34123.rjw@sisk.pl>  <20100113235010.GF24818@basil.fritz.box>
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Date: Wed, 13 Jan 2010 18:15:36 -0600
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On Thu, 2010-01-14 at 00:50 +0100, Andi Kleen wrote:
> On Wed, Jan 13, 2010 at 10:57:34PM +0100, Rafael J. Wysocki wrote:
> > On Wednesday 13 January 2010, Américo Wang wrote:
> > > On Mon, Jan 11, 2010 at 6:32 AM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
> > > > This message has been generated automatically as a part of a report
> > > > of recent regressions.
> > > >
> > > > The following bug entry is on the current list of known regressions
> > > > from 2.6.32.  Please verify if it still should be listed and let me know
> > > > (either way).
> > > >
> > > >
> > > > Bug-Entry       : http://bugzilla.kernel.org/show_bug.cgi?id=15041
> > > > Subject         : Pagemap endless read loop with LTP
> > > > Submitter       : Andi Kleen <andi@firstfloor.org>
> > > > Date            : 2010-01-10 2:09 (1 days old)
> > > > References      : http://marc.info/?l=linux-kernel&m=126308941423848&w=4
> > > >
> > > 
> > > According to Andi's later reply, it shoud not be an endless loop,
> > > it just takes a rather long time.
> > 
> > Andi?
> 
> It'll try to read 47 bits worth of 0s on 64bit x86-64. 
> 
> I haven't tried to wait until that finishes, but I estimate a few months of 
> CPU time at least.
> 
> The interface is just misdesigned, but I guess we cannot do
> anything about that for now.

It's perfectly sensible. What's not sensible is reading the entirety of
everything you find in /proc, something that just about every Linux
admin figures out moments after running their first recursive grep.

>  LTP will just need to do a workaround.

Or they could actually, you know, add a test of pagemap. Not much chance
of that, though - CVS reports it took them until 2005 to figure out that
skipping /proc/kcore was a good idea.

-- 
http://selenic.com : development and support for Mercurial and Linux


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