linux-kernel - Re: [PATCH v4 8/10] mm/hwpoison: fix memory failure still hold reference count after unpoison empty zero page

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Date:	Mon, 26 Aug 2013 16:31:50 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andi Kleen <andi@...stfloor.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Tony Luck <tony.luck@...el.com>, gong.chen@...ux.intel.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 8/10] mm/hwpoison: fix memory failure still hold
 reference count after unpoison empty zero page

On Tue, 27 Aug 2013 07:26:04 +0800 Wanpeng Li <liwanp@...ux.vnet.ibm.com> wrote:

> Hi Naoya,
> On Mon, Aug 26, 2013 at 11:45:37AM -0400, Naoya Horiguchi wrote:
> >On Mon, Aug 26, 2013 at 04:46:12PM +0800, Wanpeng Li wrote:
> >> madvise hwpoison inject will poison the read-only empty zero page if there is 
> >> no write access before poison. Empty zero page reference count will be increased 
> >> for hwpoison, subsequent poison zero page will return directly since page has
> >> already been set PG_hwpoison, however, page reference count is still increased 
> >> by get_user_pages_fast. The unpoison process will unpoison the empty zero page 
> >> and decrease the reference count successfully for the fist time, however, 
> >> subsequent unpoison empty zero page will return directly since page has already 
> >> been unpoisoned and without decrease the page reference count of empty zero page.
> >> This patch fix it by decrease page reference count for empty zero page which has 
> >> already been unpoisoned and page count > 1.
> >
> >I guess that fixing on the madvise side looks reasonable to me, because this
> >refcount mismatch happens only when we poison with madvise(). The root cause
> >is that we can get refcount multiple times on a page, even if memory_failure()
> >or soft_offline_page() can do its work only once.
> >
> 
> I think this just happen in read-only before poison case against empty
> zero page. 
> 
> Hi Andrew,
> 
> I see you have already merged the patch, which method you prefer? 
> 

Addressing it within the madvise code does sound more appropriate.  The
change which
mm-hwpoison-fix-memory-failure-still-holding-reference-count-after-unpoisoning-empty-zero-page.patch
makes is pretty darn strange-looking at at least needs a comment
telling people what it's doing, and why.

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