linux-kernel - Re: [RESEND Patch] kcore: remove its pointless size

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Message-Id: <20090717152955.6585cbf9.akpm@linux-foundation.org>
Date:	Fri, 17 Jul 2009 15:29:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	ebiederm@...ssion.com, xiyou.wangcong@...il.com, tao.ma@...cle.com,
	linux-kernel@...r.kernel.org, adobriyan@...il.com,
	mtk.manpages@...il.com, y-goto@...fujitsu.com
Subject: Re: [RESEND Patch] kcore: remove its pointless size

On Thu, 2 Jul 2009 09:41:38 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Wed, 1 Jul 2009 17:12:49 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Wed, 01 Jul 2009 16:25:05 -0700 ebiederm@...ssion.com (Eric W. Biederman) wrote:
> > > Which is better than showing a random number of dubious relationship
> > > to the size we normally show.  That code is just a maintenance problem.
> > 
> > Well it's not just that st_size is wrong before the first read.  It's
> > also wrong after memory hot-add, up until the next read.
> > 
> And I found kclist_add() is not called at memory hotplug...
> 
> 
> > > > If so, should we run get_kcore_size() in proc_kcore_init(), perhaps?
> > > >
> > > > In fact, do we need to run get_kcore_size() more than once per boot? 
> > > >
> > > > AFAICT we only run kclist_add() during bootup, so if proc_kcore_init()
> > > > is called at the appropriate time, we can permanently cache its result?
> > > >
> > > > In which case get_kcore_size() and kclist_add() can be marked __init.
> > > >
> > > > Maybe that's all wrong - I didn't look terribly closely.
> > > 
> > > Memory hot add I expect is the excuse.  There is more that could be
> > > done.  But this patch is an obvious bit of chipping away nonsense
> > > code.
> > 
> > We have the infrastructure to get this right, I think:
> > 
> > - run
> > 
> > 	proc_root_kcore->size = get_kcore_size(...)
> > 
> >   within proc_kcore_init()
> > 
> yes, seems sane.
> 
> 
> > - register a memory-hotplug notifier and each time memory goes online
> >   or offline, rerun
> > 
> > 	proc_root_kcore->size = get_kcore_size(...)
> > 
> yes. and we need kclist_add() under memory hotplug.
> 
> 
> > - stop running get_kcore_size() within read_kcore().
> > 
> > I suspect that read_kcore() will not behave well if a memory hotplug
> > operation happens concurrently.  But that's a separate problem.
> > 
> > (hopefully cc's some memory-hotplug people)
> > 
> Maybe no problem. I don't think people does memory hotplug while he reads
> /proc/kcore. (It sounds like modify coredump while investigating it.)
> 

I think I'm about to forget about the above issues.  If everyone else
does the same, they won't get addressed.  Oh well.

And I still need to decide whether
kcore-fix-proc-kcores-statst_size.patch fixes things up sufficiently
well to justify merging it.

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