linux-kernel - Re: [PATCH 1/2] mm/hwpoison: fix fail to split THP w/ refcount held

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Message-ID: <20150810085047.GC21282@hori1.linux.bs1.fc.nec.co.jp>
Date:	Mon, 10 Aug 2015 08:50:47 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Wanpeng Li <wanpeng.li@...mail.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm/hwpoison: fix fail to split THP w/ refcount held

On Mon, Aug 10, 2015 at 04:29:18PM +0800, Wanpeng Li wrote:
> Hi Naoya,
> 
> On 8/10/15 4:10 PM, Naoya Horiguchi wrote:
> >On Mon, Aug 10, 2015 at 02:32:30PM +0800, Wanpeng Li wrote:
> >>THP pages will get a refcount in madvise_hwpoison() w/ MF_COUNT_INCREASED
> >>flag, however, the refcount is still held when fail to split THP pages.
> >>
> >>Fix it by reducing the refcount of THP pages when fail to split THP.
> >>
> >>Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> >It seems that the same conditional put_page() would be added to
> >"soft offline: %#lx page already poisoned" branch too, right?
> 
> PageHWPoison() is just called before the soft_offline_page() in
> madvise_hwpoion(). I think the PageHWPosion()
> check in soft_offline_page() makes more sense for the other
> soft_offline_page() callsites which don't have the
> refcount held.

What I am worried is a race like below:

  CPU0                              CPU1

  madvise_hwpoison
  get_user_pages_fast
  PageHWPoison check (false)
                                    memory_failure
                                    TestSetPageHWPoison
  soft_offline_page
  PageHWPoison check (true)
  return -EBUSY (without put_page)

It's rare and madvise_hwpoison() is testing feature, so this never causes
real problems in production systems, so it's not a big deal.
My suggestion is maybe just for code correctness thing ...

Thanks,
Naoya Horiguchi--
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