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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

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.

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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