linux-kernel - Re: [PATCH 3/6] mm/hwpoison: fix num_poisoned

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Date:	Thu, 22 Aug 2013 13:00:37 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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 3/6] mm/hwpoison: fix num_poisoned_pages error statistics
 for thp

On Thu, Aug 22, 2013 at 12:43:08PM -0400, Naoya Horiguchi wrote:
> On Thu, Aug 22, 2013 at 05:48:24PM +0800, Wanpeng Li wrote:
> > There is a race between hwpoison page and unpoison page, memory_failure 
> > set the page hwpoison and increase num_poisoned_pages without hold page 
> > lock, and one page count will be accounted against thp for num_poisoned_pages.
> > However, unpoison can occur before memory_failure hold page lock and 
> > split transparent hugepage, unpoison will decrease num_poisoned_pages 
> > by 1 << compound_order since memory_failure has not yet split transparent 
> > hugepage with page lock held. That means we account one page for hwpoison
> > and 1 << compound_order for unpoison. This patch fix it by decrease one 
> > account for num_poisoned_pages against no hugetlbfs pages case.
> > 
> > Signed-off-by: Wanpeng Li <liwanp@...ux.vnet.ibm.com>
> 
> I think that a thp never becomes hwpoisoned without splitting, so "trying
> to unpoison thp" never happens (I think that this implicit fact should be
> commented somewhere or asserted with VM_BUG_ON().)

> And nr_pages in unpoison_memory() can be greater than 1 for hugetlbfs page.
> So does this patch break counting when unpoisoning free hugetlbfs pages?

Sorry, the latter part of this remark was incorrect. Please ignore it.

- Naoya
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