linux-kernel - Re: [RFC][PATCH 0/4] memcg: add support for hwpoison testing

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Date:	Tue, 1 Sep 2009 11:32:14 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"menage@...gle.com" <menage@...gle.com>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC][PATCH 0/4] memcg: add support for hwpoison testing

On Tue, 1 Sep 2009 10:25:14 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:
> > 4. I can't understand why you need this. I wonder you can get pfn via
> >    /proc/<pid>/????. And this may insert HWPOISON to page-cache of shared
> >    library and "unexpected" process will be poisoned.
> 
> Sorry I should have explained this. It's mainly for correctness.
> When a user space tool queries the task PFNs in /proc/pid/pagemap and
> then send to /debug/hwpoison/corrupt-pfn, there is a racy window that
> the page could be reclaimed and allocated by some one else. It would
> be awkward to try to pin the pages in user space. So we need the
> guarantees provided by /debug/hwpoison/corrupt-filter-memcg, which
> will be checked inside the page lock with elevated reference count.
> 

memcg never holds refcnt for a page and the kernel::vmscan.c can reclaim
any pages under memcg whithout checking anything related to memcg.
*And*, your code has no "pin" code.
This patch sed does no jobs for your concern.

I recommend you to add
  /debug/hwpoizon/pin-pfn

Then,
	echo pfn > /debug/hwpoizon/pin-pfn
        # add pfn for hwpoison debug's watch list. and elevate refcnt
	check 'pfn' is still used.
 	echo pfn > /debug/hwpoison/corrupt-pfn
	# check 'watch list' and make it corrupt and release refcnt.
or some.

memcg is not for pinning a page and never do such.

Thanks,
-Kame

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