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

From: KAMEZAWA Hiroyuki
Date: Mon Aug 31 2009 - 22:36:48 EST

On Tue, 1 Sep 2009 10:25:14 +0800
Wu Fengguang <fengguang.wu@xxxxxxxxx> 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

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.


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