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Date: Tue, 1 Sep 2009 14:46:52 +0800
From: Wu Fengguang <fengguang.wu@intel.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>, Andi Kleen <andi@firstfloor.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>,
       "lizf@cn.fujitsu.com" <lizf@cn.fujitsu.com>,
       "nishimura@mxp.nes.nec.co.jp" <nishimura@mxp.nes.nec.co.jp>,
       "menage@google.com" <menage@google.com>, linux-mm <linux-mm@kvack.org>
Subject: Re: [RFC][PATCH 0/4] memcg: add support for hwpoison testing
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References: <20090831102640.092092954@intel.com> <20090901084626.ac4c8879.kamezawa.hiroyu@jp.fujitsu.com> <20090901022514.GA11974@localhost> <20090901113214.60e7ae32.kamezawa.hiroyu@jp.fujitsu.com>
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On Tue, Sep 01, 2009 at 10:32:14AM +0800, KAMEZAWA Hiroyuki wrote:
> On Tue, 1 Sep 2009 10:25:14 +0800
> Wu Fengguang <fengguang.wu@intel.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.

We grabbed page here, which is not in the scope of this patchset:

        static int try_memory_failure(unsigned long pfn)
        {      
                struct page *p;
                int res = -EINVAL;

                if (!pfn_valid(pfn))
                        return res;

                p = pfn_to_page(pfn);
                if (!get_page_unless_zero(compound_head(p)))
                        return res;

                lock_page_nosync(compound_head(p));

                if (hwpoison_filter(p))
                        goto out;

                res = __memory_failure(pfn, 18,
                                       MEMORY_FAILURE_FLAG_COUNTED |
                                       MEMORY_FAILURE_FLAG_LOCKED);
        out:
                unlock_page(p);
                return res;
        }

> 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.

Looks like a good alternative. At least no more memcg dependency..

Cheers,
Fengguang
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